Remove FLAG_INPUTFILTER_TRUSTED in MotionEventInjector

All input events coming into inputfilter from
InputDispatcher will now have FLAG_INPUTFILTER_TRUSTED set by default.
It is now up to the inputfilter (a11y) to remove this flag when appropriate.
One such case is where an unknown a11y service is injecting the events.

From auditing the a11y code, the only such place is MotionEventInjector.

Therefore, label all events coming out of MotionEventInjector as
"untrusted" by removing this new flag INPUTFILTER_TRUSTED.

Separately, a11y should be distinguishable from regularly injected input
events. To tell apart the events injected by a11y, add a new flag,
FLAG_INJECTED_FROM_ACCESSIBILITY. This flag requests InputDispatcher to
apply the device id = -2 for the injected events.

Test: atest CtsInputTestCases:android.input.cts.GamepadWithAccessibilityTest
Test: atest MotionEventInjectorTest
Bug: 175069843
Change-Id: I12d4a7bd6fbab8af202f5ae88b6be97ff9e1754c