Make sure home activity gets resumed after started.

When there is only one activity and it gets paused, activity manager will
try resuming some other activity. If there is no other activity, it will
start home activity and try to resume it. This will fail, because the
activity stack doesn't want to enter recursive resuming. We need to
detect this situation and schedule additional resume of top activity
after home gets started.

Bug: 21500945
Change-Id: I6ca703f8d97bcc4001e7da4cd5a5db63bfb5c861