FWB: Forward port long-press-back to kill app (2/2)

Change-Id: I912cabd737f7550fd10b51492e3e5a8643e304fd
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.

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