Add system wide long-press back button functionality

This CL adds long-press back button functionality as a system
intercepted keypress.

Incorporated in this is an update to clockwork overlays so that
long-press on back will launch ACTION_VOICE_ASSIST.

BUG: 27313719
BUG: 27427088
Change-Id: Iaff337dc50e0022d2392c19d095cec98563b1c7d