Add panic detection to back button

Adds "panic" detection to the back button.  Implemented solution
uses 4x button presses in a short duration to detect for "panic".
The value used to determine the duration between key up and key down
that still count as a multi-button press is configurable via the
Settings Provider.

BUG: 28027764

Change-Id: Ibf1370ff3cb539a9a54002a8704922744a3ca5d7