Pass the values of the MotionEvent instead of the event instance itself

to BackAnimation.

The original MotionEvent instance may have been recycled by the time BackAnimation processes it, since BackAnimation runs in a different thread. This could cause crashes from both Java side and native side, as described in b/233163975#comment11.

We work around this by passing the numerical values that BackAnimation
need instead of the MotionEvent itself.

Test: Swipe back in various scenarios. Make sure back can be triggered.
Try cancel. Make sure back can be cancelled.
Test: atest BackAnimationControllerTest
Bug: 233163975

Change-Id: I4792debc705536b49d1caafcc43e5466941c1d43
Merged-In: I4792debc705536b49d1caafcc43e5466941c1d43