Single-device prediction only

After further discussion about the potential usage of the
MotionPredictor API, we decided that returning a List<MotionEvent> is
inconvenient for the application.

Update the native MotionPredictor to return a nullable MotionEvent
instead.

Bug: 167946763
Test: native
Change-Id: Ib1382431d894b9561d9a9e2e96c360e2b7a19b5c