Make all activities on visible and focusable leaf tasks be resumed

Currently, only the top-most activity of a root task can be resumed,
no matter how many descendant tasks or their windowing modes are.

Resuming the top activities of all visible and focusable leaf tasks
to be more flexible and can support features like app-pairs.

Therefore, Task#mResumedActivity is now set at leaf task vs. root task.
And Task#getResumedActivity() now returns the resumed activity of the
top-most leaf task. Same as Task#mPausingActivity.

Bug: 172885410
Test: presubmit
Change-Id: Id0a73b39afd03afb2b9572752e98adce8de1619b