Generate key events for stylus button presses

Ensure all devices that report stylus buttons are categorized as a
keyboard. This means that KeyboardInputMapper will get a chance to
handle the events from these devices.

Then, we allow KeyboardInputMapper to process stylus button events.
Since stylus button (e.g. BTN_STYLUS) are mapped to Android key codes in
the Generic.kl file, KeyEvents with the appropriate key codes will be
generated for these button presses.

Bug: 246394583
Test: atest inputflinger_tests
Change-Id: I92d4a60f23f98f9d239edf1f4dd400e6e528e350