SystemUI: make long pressing recent switch to last app

With this patch, long pressing the recents key on the navigation bar
will attempt to switch to your last application in the activity stack
before the active one.

The user's current home launcher package is ignored as well as SystemUI (so that
RecentsActivity does not influence the behavior).

Change-Id: I38771e0fce16b55bb5186af47115b4e812cb60b0
Signed-off-by: Roman Birg <roman@cyngn.com>