Ensure pointer location view is attached to the policy thread.

Previously we created the PointerLocationView on whatever thread
happened to trigger the call to updateSettings().  There was
also some messiness around having to add or remove the view
while not holding mLock.

Now, just post the work to the policy handler.

This also makes it possible for us to use invalidate() instead
of postInvalidate() in PointerLocationView, which is more efficient.

Change-Id: I0646d7aeecffdc22f6ac56ae3ef951e7a12e2b93