Communal Profile Notifications

Allows notifications (including media, as well as regular) of the
communal profile to work, both from the lockscreen and when actively
using the current user.

Test: atest com.android.server.pm.UserManagerTest
Test: atest SystemUItests:com.android.server.pm.NotificationLockscreenUserManagerMainThreadTest
Test: atest SystemUItests:com.android.systemui.statusbar.NotificationLockscreenUserManagerTest#testGetCurrentProfiles
Bug: 309168621
Flag: ACONFIG android.multiuser.support_communal_profile DISABLED
Change-Id: I87cd7e625b362961f78dab0f789ea6b51d985099