Ensure stylus buttons generate events consistently

Add tests to verify that a touchscreen stylus and fused/unfused external
styluses generate button events consistently.

Make changes in TouchInputMapper to enforce consistentcy for external
stylus buttons. In particular, we allow the external stylus's button
state to be applied to the touches even when there is no fusion in
progress.

Bug: 246394583
Test: atest inputflinger_tests
Change-Id: I645481db18e1bec8b61d8309aa4f02afcf1a5383