Replace SharedFlow with StateFlow to fix notif height

When redacted notifications become visible by face unlock,
a flurry of change events are sent. The MutableSharedFlow
that was processing these events became overloaded and
it was dropping events necessary to process the change.
This resulted in the notifications overlapping the lock
icon.

Use a StateFlow for better performance and to prevent
dropped updates.

Fixes: 338602791
Test: atest SharedNotificationInteractorTest
Test: manual - race condition with redacted notifications,
unlock many times with face on keyguard
Flag: ACONFIG com.android.systemui.migrate_clocks_to_blueprint
TEAMFOOD

Change-Id: I3190881b81f68dc1e88b927ce62e64a9d7e12a6b