Map new home and back keyboard shortcuts

Back: META+~, META+DEL (mapped to KEYCODE_BACK in InputDispatcher), META+DPAD_LEFT
Home: META+H, META+ENTER (mapped to KEYCODE_HOME in InputDispatcher)

Fixes: 246660997
Fixes: 258853738
Test: manual
Change-Id: I6691b903c9e247ab6924d1ddb202991c7b0df1f2