Introduce AccessibilityService#onMotionEvent()

This API allows accessibility services to intercept motions events, to
expand device control capabilities to more types of input devices
such as watch rotary wheels and gamepad joysticks.

Unlike KeyEvents which are optionally consumed, for MotionEvents the
service must declare the event sources it wishes to consume and then
events from these sources are *always* consumed. The source list can be
changed by the service at runtime. By always consuming the requested
events we avoid the potential "wait and see" latency concerns that
onKeyEvent deals with.

Bug: 247550565
Test: atest AccessibilityEndtoEndTest
Test: use in an a11yservice to observe joystick events
Change-Id: Iea54b00b21f5348f50d94d6e2213df24c0c8a36a