Add @SystemApi registerContentObserverForAllUsers in ContentResolver

Also add ContentObserver#onChange(boolean, Collection, int, UserHandle) so
clients can know the corresponding UserHandle for the notification.

Bug: 203606981
Test: atest CtsContentTestCases and manual testing to verify the observer can get notifications for
all the users.

Change-Id: Icec7e70b71d7de1c15faf1736539f169856304a8