Add NotifCollection dismiss methods

Now NotifCollection has three methods for sending dismissals to system
server:
1. dismissNotification
	- dismisses a single notification
	- use case: when user swipes away a single notification
	- ends up calling IStatusBarService#onNotificationClear to
	update system server
2. dismissNotifications - dismisses multiple notifications it's passed.
	- use case: user dismisses all 'gentle/silent' notifications
	- ends up calling dismissNotification (#1) multiple times
3. dismissAllNotification
	- dismisses all clearable notifications associated with a given userId
	- use case: user clicks on "clear all" to clear all their notifications
	- ends up calling IStatusBarService#onClearAllNotifications to
	update system server

Test: atest NotifCollectionTest
Test: atest SystemUITests
Change-Id: I1fdf38fd192824b1dfc45dd3a5e1fea0a51e2e75
Fixes: 149241859