Revive DeadZone

Override dispatchTouchEvent for the root FrameLayout
of NavigationBar to process ACTION_OUTSIDE MotionEvents
and dispatch directly to DeadZone to keep track of the
most recent outside tap.
Clarified documentation of ACTION_OUTSIDE behaviour.

Bug: 37552674
Test: open IME, tap any key, then quickly tap on top half
of the home button. The home button tap is ignored
and device does not go to homescreen.

Change-Id: Icb5cf6c76959f3514b8b94c09e38cc5434f31b23