Make activity on visible and focusable leaf tasks be resumed

RuntimeExceptoin throw because of performing pause of activity that
is not resumed. This CL make sure the activity be resumed on visible
and focusable leaf tasks.

Bug: 177932048
Test: 1. use bubbles test app
      2. add some bubbles
      3. tap between bubbles, collapse stack, open stack
Change-Id: I571cb6d535736cc8dff70583cffa3c8a867c5722