We were sort of using observers to maintain the new message notifications.
However, other parts of the code would poke into the notification controller
to set things such as a list of newly added message IDs. Now, we rely
exclusively on db observers to manage notifications.
As a side effect of this, we now set the notification text correctly to be
the most recently _added_ message. This may be different than the most recently
sent message [since there may be a non-negligable delta between when the
message was sent and when it was received].
NOTE this still suffers from an outstanding bug where we continue to get
notifications when the Eamil UX is visible. That and monitoring changes to the
account table will be addressed in future CLs.
Change-Id: I4c68273716cc685574a1ca71e5d634f53fe0d882
83693a6aca