Exit dream on MotionEvent.ACTION_UP.

Previously, DreamService would exit when any
MotionEvent was detected. This led to dreams
ending while the user's finger was still on
the screen.

This change adjusts the exit action to be on
ACTION_UP to correspond with the finger lifting
off the screen, matching user expectation.

Bug: verified non-interactive dream exits on
     lifting finger.
Test: 210919013
Change-Id: I767cdb179f99073c1fffc66fe492bdcbbae599c2