Changed task focused to be based on focused app vs. window

Task#isFocused and onWindowFocusChanged (renamed to onAppFocusChanged)
where based on the current focused window which is wrong since not all
windows are contained within a task. Changed to use DC#mFocusedApp
instead which tracks focus changes to app windows.

Also,
- Start calling dispatchTaskInfoChangedIfNeeded in
Task#onAppFocusChanged
- Removed DC#mLastFocus that isn't used.

Bug: 180525887
Test: Task focus doesn't change when notification shade is expanded.
Change-Id: I1abf97b5f8d7eedd36bd429835094f6a48361bee