Finish already paused activity if it should be finished after pausing

If the app requests its activity to be finished allow it to be finished
immediately if the activity is already paused.

Also, don't schedule stop for an activity that was already finished in
ActivityStack.stopActivityLocked().

Bug: 22170595
Bug: 22397812
Change-Id: If349f7d3070a1fd2c0688fda7381b42f625ed9e4