Clean up content observer code.

Improved the documentation a little bit.

Fixed a bug in ContentService wherein if a ContentObserver was
passed as an argument and its deliverSelfNotifications() method
returned true, then notifyChange would tell all observers that
the change was a self-change even though it was only a self-change
from the perspective of the provided observer.

Deprecated ContentObservable.notifyChange since it is never
used and in general it shouldn't be because we want the notification
to be posted to the handler.

Change-Id: Idde49eb40777e011a068f2adae8a32f779dfb923