Fix setPrivateNotificationsAllowed

1) Add an on-changed broadcast so the value takes effect immediately
2) initialize NotificationLockscreenUserManagerImpl earlier, before its
dependencies need it
3) Make the behavior match the name - it should redact notifications,
not hide them

Also made some small changes in NotificationManagerServiceTest to fix
test failures on -eng builds.

Test: NotificationLockscreenUserManagerTest
Test: NotificationManagerServiceTest
Test: view notification history
Test: test app that calls setPrivateNotificationsAllowed
Flag: aconfig android.app.keyguard_private_notifications
Bug: 309920145

Change-Id: If73c2cbf02c234261bbc84cc02e7ba6a5187be2e