Split NotificationUserManager out of StatusBar.

This class handles functionality related to private/public contents,
redacting notifications, whether to show notifications on the
keyguard, and user changes.

Bug: 63874929
Bug: 62602530
Test: runtest systemui
Test: Compile and run
Change-Id: If44fdf5f89c1e72c4bd5f49731fd4a158ea50552