Avoid NPE when stop() is called inside callback

stop can be called inside onMotionChange. Since that mLastAcceleration
is null and it makes NPE when mUpdateRunnable is being excuted next
time.

Change-Id: I7ed276ef63eb7fcb8fcf4dedc53eba0b13fdc539
Signed-off-by: Alan Jeon <skyisle@gmail.com>