Develop Reference

Where are Full Bleed Layer image icons used?

My question is:
When creating an app icon, something called Full Bleed Layers is created as well and I don't know where this icon will be in use.

Full bleed layers = the base adaptive icon, which is being masked (it is only being shown masked).
The link in the SDK documentation is broken: see Designing Adaptive Icons.

What Design is Material?

I have a TabLayout which it's height is 50dp. each tab inside it contains both icon and title inside itself. But as this article says If I use both icon and title inside a tab, TabLayout height should be 72dp. now my question is :
if I set 50dp for my TabLayout's height, Will go out my design from Material Design? Thanks
sorry about my English.

All sizes mentioned in Material.io are required to respect Material Design Guideline.

Well, Material Design is just a set of guidelines based on some research. It suggests good practices and consistent solutions. Material Design guidelines used to be pretty strict, for example when it comes to shapes of buttons, because they wanted to make users used to some concepts. With Material Design 2 the guidelines relaxed a lot.
Minimal size is meant to keep items distinguishable and accessible, but still not too big. I wouldn't make tabs much smaller, because it may look squashed. It won't affect the usability though as minimal size for clickable items is 40dp, I guess. You would probably need to reduce spacings and sizes of other items in your app in order to make it look consistent with shorter tabs.
Changing font is fine as long as it was designed for on-screen reading. The size can be any as well as long as text is large enough to be readable.
If your design is consistent, items are distinguishable, easy to use and aligned well, then you're good. Take a look at Material Studies on material.io - these designs show how far you can go without breaking anything.

Get rid of the margin on Image Asset for launcher icons on Android Studio

I am trying hard to get rid of the extra margin around the ic launcher icons generated by Android Studio without luck. I know this is one of Google's material design trends. Though, the icons are so small on devices which is ugly.
I have Googled for around three hours and the only workaround I could find is to generate the clipart with a transparent background and photoshop the background with rounded corners which is pretty slow.
Is there a better solution than this?
Thank you.

Per the Designing adaptive icons blog post, the adaptive icon images created in the drawable directory for use on API 26+ devices follow the adaptive icon size and shape:
You'll note that while the entire image must be 108dp by 108dp, the actual visual area is only the center 72dp - the outer edge is only seen when dragging the icons around (where parallax effects may cause it be visible).
The Pixel Launcher, as well as many third party launchers, enforce the use of adaptive icons on API 26+ devices, so you should always design with that requirement in mind - the alternative, in the Pixel Launcher's case for instance, is to place your non-adaptive icon within a white circle.
Therefore you should always design your icon such that the background layer takes up the full size, but any foreground image you put on top of that background layer should be contained within that center area.
You can't just remove that margin without the foreground image being messed up, but as per the Implementing Adaptive Icons follow up blog post, you can remove any white excess around the foreground by putting your trimmed image within a transparent drawable of 108dp x 108dp by using an InsetDrawable.
<!-- Center a 54x54dp image in the 108x108dp size of adaptive icons -->
<inset
android:drawable="#mipmap/ic_fg_trimmed"
android:insetLeft="25%"
android:insetTop="25%"
android:insetRight="25%"
android:insetBottom="25%" />
Although this would have to be something you'd need to do to the ic_launcher_foreground image yourself after the fact.
A better solution, in most cases, is to use vector drawable images as the input to the Android Studio Image Asset wizard - this will ensure they are always the right size and don't take up any additional space in your APK.

I think i had the same problem as you. I solved this with a nasty hack, in the AndroidManifest.xml I just changed the roundIcon to point to the standard icon.
<application
…
android:icon="#mipmap/ic_launcher"
android:roundIcon="#mipmap/ic_launcher"
See more from the docs.

How big should Action Bar(Toolbar) icons be in the new Android Material design?

I tried searching the design guidelines, but I couldn't find anything related to this. I am trying to figure out the optimal size for Action bar icons in the new L release. Here is an example of what I am trying to achieve:
I tried the previous 48dp size, but it looks a lot bigger than the ones in that picture.

From Material Design Keylines Touch Target Size, size for icons is 24dp

Full icon size should be 24dp with a "live" area of 20dp.
See this answer: https://stackoverflow.com/a/14908467/3979479

Action Bar with centered logo

I would like an ActionBar where I place an image/logo in the center of the bar, rather than the default left end. I'm using the standard android classes building with level 18, no compatibility classes and no ABS. I've looked at this solution which seems to be the most popular answer:
ActionBar logo centered and Action items on sides
... but it simply doesn't work for me. I'm using various devices including HTC One with 4.2.2, Nexus 7 with 4.2.1. However much I play with the solution, the text or image still appears left aligned, plus the layout file only takes effect for the width of the image, and does not span the entire ActionBar.
My request is just for the solution to centering the logo, I'm not interested in any left or right home/menu buttons.

A centered logo on the ActionBar actually violates the Android Application Design Guidelines unfortunately, you can find a comprehensive guide here:
http://developer.android.com/design/get-started/ui-overview.html
The accepted norm is to have the logo to the left hand side.