Suppress notifications when device enter lockdown

This CL makes the following modifcations:
1. Add LockPatternUtils.StrongAuthTracker to monitor
the lockdown mode status of the phone.
2. Call mListeners.notifyRemovedLocked with all the
notifications in the mNotificationList when entering
the lockdown mode.
3. Call mListeners.notifyPostedLocked with all the
notifications in the mNotificationList when exiting
the lockdown mode.
4. Dismiss the function calls of notifyPostedLocked,
notifyRemovedLocked, and notifyRankingUpdateLocked
during the lockdown mode.

The CL also adds corresponding tests.

Bug: 173721373
Test: atest NotificationManagerServiceTest
Test: atest NotificationListenersTest
Test: manually verify the paired device cannot receive
notifications when the host phone is in lockdown mode.
Ignore-AOSP-First: pending fix for a security issue.

Change-Id: I7e83544863eeadf8272b6ff8a9bb8136d6466203