SystemUI: survive notification update spam

When handling a very high number of incoming notification updates, SystemUI can
choke up at updateNotifications() in PhoneStatusBar. Funnel
updateNotification() calls through the StatusBar's handler and don't
stack them up.

Change-Id: I806d1fd8eac73c4af0820319d127423ae6467f60
Signed-off-by: Roman Birg <roman@cyngn.com>
(cherry picked from commit d999962e)