Add comments about OS bugs that need working around.
Refactor to simplify buildNotification()
Java code style: indent, don't align method formal parameters
Workaround: Hide Paused notifications on API < 21

TODO: Hide the activity's Stop on API < 21, thus completely hiding the
distinction between Paused and Stopped which is only for Paused lock screen
notifications on API 21 Lollipop.