InputVerifier: verify button events and states (attempt 2)

Check the following things:

* BUTTON_PRESS and _RELEASE actions have a single, valid action button
* No redundant BUTTON_PRESS or _RELEASE actions (i.e. for buttons that
  are already pressed or released)
* Button state remains consistent throughout the stream, i.e.:
  * Buttons are only added to the state by BUTTON_PRESS (though DOWN
    events can have "pending" buttons on them)
  * Buttons are only removed from the state by BUTTON_RELEASE
  * When a DOWN event has pending buttons in its state, it is
    immediately followed by a BUTTON_PRESS for each one

We could also verify that press and release events for primary,
secondary, and tertiary buttons are accompanied by down and up events.
However, I couldn't find any documentation that states which buttons
should result in down or up events, so I haven't implemented this for
now.

This is the second attempt to land this change, due to the original
causing test failures. Change I5c259c13d433d3010d2cf9c6fe01e08ba5990ef7
fixes the failures. v2 also adds a separate flag for button state
verification, as it is actually used in production to check events
injected by assistive technologies, whether or not the flag that I
previously thought was guarding it is enabled.

Test: connect a mouse and a touchpad, enable the verifier, play around
      with the buttons, and check that any issues found by the verifier
      appear to be legitimate. (I found b/391298464 , and checked that
      the verifier caught a button problem with a partial fix to
      b/372571823 .)
Test: atest --host libinput_rust_tests
Test: atest --host frameworks/native/libs/input/tests/InputVerifier_test.cpp
Test: atest --host \
      frameworks/native/services/inputflinger/tests/InputDispatcher_test.cpp
Bug: 392870542
Flag: com.android.input.flags.enable_button_state_verification
Change-Id: I46f489b26df8785563e41e58135b6b5de4ff62a2