Remove mFocusedActivity in AM and only keep focused stack

- Remove mFocusedActivity and replace it with resumed activity
  where applicable.

- Remove setFocusedActivityLocked and related recursions, only
  keep track of focused stack.

- Update focus app with WM at resume time.

- Some clean up of startActivityUnchecked, make sure each of the
  setTaskFromXXX methods set up the focused stack properly, and
  remove the extra setFocusedActivityLocked before starting.

bug: 29937415

Change-Id: Ib93d3f4d9cded0aa3b480d0d423435c536b7e7b5