Keep app focus and input focus current

Change ag/422870 mistakenly set app focus to null in the
InputDispatcher. This change restores setting app focus correctly.

In addition, there were a few places where window focus
(i.e. mCurrentFocus) could be modified without notifying the
InputDispatcher. This change fixes those locations.

Should fix 14564396.

Change-Id: Ie24f3410fdd1f5f1eefbce02759fef4149e59620