Enable user graularity for lockdown mode

The NotificationManagerService registers a LockPatternUtils.StrongAuthTracker
to observe the StrongAuth changes of every user.
More specifically, it’s the STRONG_AUTH_REQUIRED_AFTER_USER_LOCKDOWN flag.
Via this flag, NotificationManagerService can perform the following operations
when the user enter or exit lockdown mode:

Enter lockdown:
1. Remove all the notifications belonging to the user.
2. Set the local flag to indicate the lockdown is on for the user.
   The local flag will suppress the user's notifications on the
   post, remove and update functions.

Exit lockdown:
1. Clear the local flag to indicate the lockdown is off for the user.
2. Repost the user’s notifications (suppressed during lockdown mode).

The CL also updates corresponding tests.

Bug: 173721373
Bug: 250743174
Test: atest NotificationManagerServiceTest
Test: atest NotificationListenersTest
Ignore-AOSP-First: pending fix for a security issue.

Change-Id: I4f30e56550729db7d673a92d2a1250509713f36d
Merged-In: I4f30e56550729db7d673a92d2a1250509713f36d
(cherry picked from commit de3b12fc)