Implement support for Android TalkBack

[IsaMorphic]

What does the pull request do?

This pull request adds a much-needed implementation of UI automation / screen reader accessibility features to the Android implementation of Avalonia. It does so by implementing Android's ExploreByTouchHelper built-in as a compatibility shim between Avalonia's own AutomationPeer API and Android's AccessibilityNodeInfo API.

What is the current behavior?

The current behavior of Avalonia on Android is to essentially operate as a dynamically rendered raster SurfaceView which is completely opaque to screen readers such as Android's TalkBack or HumanWare's KeySoft. This is completely unsuitable for users who have visual and/or cognitive impairments and need a screen reader to help them understand what's on the screen.

What is the updated/expected behavior with this PR?

The new behavior that this PR implements is a fully functional prototype integration between Avalonia and Android's various accessibility APIs. This integration includes support for all relevant specialized "providers" within the AutomationPeer API and creates a performant, responsive, and verbose user experience for visually impaired users of Avalonia apps on Android.
To test, enable TalkBack on your Android mobile device and run the ControlCatalog sample. You will observe that the "explore by touch" feature now responds appropriately to user input and allows them to hover over interactive elements like buttons and text fields in order to focus them and have their attached labels read aloud.

How was the solution implemented (if it's not obvious)?

The solution was implemented by implementing a specialized AccessibilityDelegateCompat via Android's ExploreByTouch abstract. This delegate acts as a plug-in for Avalonia's existing automation features into Android's accessibility APIs designed for custom controls.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

This PR only modifies internal Avalonia.Android functionality and as such does not expose any new APIs or make changes to the existing API surface.

Fixed issues

• Related to previously closed Accessibility support for targeted platforms needs to be considered #585

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[cla-avalonia]

• All contributors have signed the CLA.

[IsaMorphic]

@cla-avalonia agree

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[maxkatz6]

Thank you!

[IsaMorphic]

You betcha! :3