Develop Reference

How to add an overlay on widget on android auto

I'm looking into Android APIs in order to understand how to add an overlay widget to Andoid Auto but to no avail.
I'm not willing to add the widget on the main phone's screen but on the car display/head unit: my purpose is to write a simple widget which overlays the android auto bottom bar and adds a very readable clock (since the embedded one is really small!!!).
I've looked into existing tools for this to no avail and can't even find any hint if this is even possible using the APIs to target specifically the second screen used by AndroidAuto.
The reasonable starting point might be the WindowManager with appropriate LayoutParameters but this does not seem to be the right way.
Any hint?

Android Auto is designed to help minimize distraction so drivers can focus on the road. This means you can't do much, unless you have access to the full Android Auto SDK. Currently, only some big companies have access to the official SDK. There's also an unofficial SDK that allows you to make your own custom apps, but since version 3.0 Android Auto no longer supports launching custom apps.

Multi Window App Development

I want to develop an android application which has to run two other applications on the device with one app on upper half of the screen and the other in lower half of the screen. Any clues/suggestions to start developing this ? Thanks in advance !

it's not possible multi-pane-layout two other apps.
if u want your app with multiple activities follow this tutorials for your references:
it's called named as multi-pane-layouts and offical introduction about multi-pane-layouts below link:
http://developer.android.com/design/patterns/multi-pane-layouts.html
and it's shows how to implement multi-pane-layouts on your project below link from android official side:
http://developer.android.com/guide/practices/tablets-and-handsets.html
and you can download example project from github below link:
https://github.com/faisalgeek/MultiPaneAndroidApp

This is not possible, except via custom device firmware or ROM mods. In standard Android, only one app can be used at a time.
However:
You are welcome to support multi-window apps on some Samsung devices, as they have extended their firmware to support this concept
You are welcome to have an activity's theme set up to not fill the screen, though anything else around the activity will at most be visible and will not respond to touch events
If all of these applications are yours, combine them into one app, and then you can display elements of that app alongside other elements of that app however you wish.

If you are looking for a feature for your app which supports multi-window then Latest Android version N has a new cool feature called Multi-window support, you can let your app support multi-window which can be resized and scaled which is exactly your requirement. you can find the reference link here

how to permanently override android soft keys behaviour and display through

I am relatively new to android and want to create an application that permanently overrides androids basic softkey behaviour and view (for devices with soft keys).
Some functionalities I want to implement are changing the size of the softkeys window at the bottom, change its images, and possibly change its functionality.
For example, the user can set the size to of the softkey to be "large", "medium" or "small". And I can change the functionality of the back button to open say a particular application instead of going "back".
I'm basically looking for a high level answer as to how to do this, a basic direction of what I should read/study in order to be able to accomplish this. I realise this may require root access.
Please note that I want this behaviour to change not only in my application but I want the effect to exist on all applications. If this requires the application to be running atleast at the backend, that is fine.
After doing some decent amount of search, it seems I will have to make changes in the systemUI.apk, or possibly get its source code and modify it. Is this correct?
Thanks in advance.

I don't think even root is going to be enough for the type of changes you are describing. You're going to need to edit the Android source code and build your own system image.

Well you can't override system resources because they are part of the system image.
What you are looking for is called Home Application which can be developed like any other android app no need for root , you can find an example for it in your sdk samples.
Home Sample Application.
your app would be responsible to have UI components to send the user to all of the phone functionalities which includes:
Place for wegits
Place for apps listing (menue)
Access telephony functionality (call, phone history ...)
Access settings.

How the Activity related to Window in Android?

In Android, the concept Window seems not important anymore.

Until recently devices running Android have generally had small displays that would not be able to present multiple "windows" in a usable way. Android best supports display of a single "window" covering the entire screen at any time - along with dialogs for convenience. That window is typically managed by the system at the command of an Activity. So yes .. in general you should forget about windows and use Activities to manage the UI.

Getting App/Widget on Homescreen?

I was wondering if it's possible to get the icon (shortcut) of your app on the home screen during installation instead of inside the standard apps folder ?
And I did find out that Android has AppWidgets but I was wondering if it supports "standard" widgets as well (javascript/html/css) ?

At the moment it doesn't support "standard" widgets (javascript/html/css), but you should take a look at the Titanium Project
And there is no way to automatically put a shortcut on the homescreen when the application is installed. Not sure if most users would like that either.