Add null check between calling onMotionPauseDetected() and Changed()

Some listeners such as ScreenPinnedInputConsumer only care about the
first pause detection and thus clear the MotionPauseDetector in
onMotionPauseDetected(). Because clear() sets mOnMotionPauseListener
to null, we need a second null check before onMotionPauseChanged().

Fixes: 169329910
Change-Id: I4fc30d67ad808a5b0c8feb10cc5392d7cae042b1