Add FLAG_IS_ACCESSIBILITY_EVENT to KeyEvent and MotionEvent

This flag indicates that the event was modified or generated by an
accessibility service.
It allows apps to tell apart real hardware events, events that are
injected (device id == -1), and events coming from accessibility (has
flag is_accessibility_event).

Events that have gone into accessibility, and got reinjected without
being modified will not be distinguishable from real hardware events.

In the next release, we will make FLAG_IS_ACCESSIBILITY_EVENT public
api. Until then, applications will have to hard-code its value (0x800)
to use it. The value is the same for both KeyEvents and MotionEvents for
convenience.

Bug: 175069843
Bug: 152399927
Test: atest VerifiedMotionEventTest VerifiedKeyEventTest
Test: atest AccessibilityGestureDispatchTest
Test: atest inputflinger_tests libinput_tests GamepadWithAccessibilityTest
Change-Id: I38ac2ab8e19e32cad927742c623f14f43ea0c588