Add Ctrl-Alt-Backspace to Back mapping

Android devices with a physical keyboard connected may not have a
way to execute the "Back Behaviour" without using a touchscreen
or pointer. This adds a ctrl-alt-backspace mapping to the
back behaviour, similar to TalkBalk.

Use cases include Pixel C, accessibility situations where
touchscreens are not convenient or feasible, or other phones/tablets
with a hardware keyboard attached and keyboard only interaction is
desired.

Previous to http://ag/3540362, ESC provided this functionality.

Bug: 71907807
Test: Manual testing on Marlin
Change-Id: I5015a17add26824a40e5eac1bced8e9ca7b98efa