Ensure that focusable stack is focused when starting a new activity.

- When starting a new activity, if the old top activity was not
  focusable the focused stack was not updated, leaving the activity
  in a non-resumed state.  Now, ensure that we focus the stack if
  starting an activity allows the stack to be focused.

Test: Enable tap-through in the SystemUI tuner, tap on the activity and
      ensure the menu shows.

Bug: 32636739
Change-Id: I8dd964baacd1e7352880ddd2b2324b947cbec556