Replace the cached MotionEvent with getX() value to improve GC state

avoid to cache the whole MotionEvent, since it will clone the whole
instance fields again to cause GC triggering, since we only need the
getX() value in the MotionEvent, rather than the whole data

Bug: 195306545
Test: make SettingsRoboTests
Change-Id: Ic3840b0a4eb6587ec229ba3de2c62ec868d1a535