Set FLAG_IS_ACCESSIBILITY_EVENT for events injected from accessibility

If an input event was modified or injected by accessibility, set the
appropriate flag. This helps applications distinguish the real hardware
events from synthesized events.

This also allows a11y services more flexibility in modifying the event
streams coming from real hardware.

Bug: 152399927
Bug: 175069843
Test: atest inputflinger_tests libinput_tests
Change-Id: I805cba4c84582fa4cd4e0892ec00428d4b255ec6