android: Add mapping from NDK Keycode to VirtualKeyCode

[torokati44]

When working on https://github.com/torokati44/ruffle-android, I discovered that all keypress events arrive with their scancode set to 0, and their keycode set to None. Which I found less than useful.

The NDK docs for AKeyEvent_getScanCode mention that "These values are not reliable and vary from device to device.".
So I guess always setting them to 0 is acceptable from their end.

Tested this on Android 12, with a keyboard connected to my computer, through wired adb and https://github.com/Genymobile/scrcpy/, and it seemed to work fine.

AFAICT This fixes #1867.

• Tested on all platforms changed
• Only Android is affected, and I am testing on it.
• Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• I have not found any existing documentation that would have to be updated due to this change.
• Created or updated an example program if it would help users understand this functionality
• No new functionality, only implementing/fixing an existing one.
• Updated feature matrix, if new features were added or implemented
• The change is not significant enough to warrant updating the matrix.

[torokati44]

I think this is now reviewable.

I have left quite a few ??? comments in there where it wasn't immediately obvious what the mapping should be.
What should I do about those? Delete those comments and accept the current mapping as a whole as "good enough", or remove any mapped keys which may not be a perfect match? Or leave the uncertainty in there with those comments?

[msiglreith]

thanks! the less clear cases are fine in my eyes

[MarijnS95]

Started playing about with these codes, and it seems you haven't implemented a mapping for KeyCode::Back which would be needed to handle a graceful app exit (or going to the previous page) when the user presses it. The implementation for VirtualKeyCode::Back reads:

winit/src/event.rs

Lines 962 to 964 in f04fa5d

	/// The Backspace key, right over Enter.
	// TODO: rename
	Back,

So I don't think we should add a new Back VirtualKeyCode and rename existing uses to Backspace, as that's really hard to notice for end-users upgrading their winit. Perhaps we can piggyback NavigateBackward but android already supports that as KeyCode::NavigatePrevious:

Keycode::Back => Some(VirtualKeyCode::NavigateBackward),

How do you think we should go forward?