frameworks: allow last app action to be assigned to hardware keys
Also fix multi-user handling for 'switch to last app' and 'kill
foreground app' actions.

Adapted to crDroid sources by @cristianomatos. Some codes was already implemented

Change-Id: I1fb586d19b1198e6888b0f11e0a2813acf0bcd5b
Signed-off-by: Roman Birg <roman@cyngn.com>
Signed-off-by: Danny Baumann <dannybaumann@web.de>