Pass MotionEvent by reference

When the object being passed isn't nullable, it should be passed by
reference rather than by pointer.

This makes it clear to the caller that the passed object should always
be valid.

Bug: 236772648
Test: TEST=libinput_tests; m $TEST && $ANDROID_HOST_OUT/nativetest64/$TEST/$TEST
Change-Id: Ib3d4ef1c4a93471dff8de53b7c7873ccf55e7266