Promote FeatureFlag best practices in notifications code & tests

* Introduces a ViewRefactorFlag object which:
  * Avoids bugs from boolean flag fields in views being uninitialized
  * Provides helpers for asserting & checking the flag state
* Removes FeatureFlags from AmbientState
* Changes all uses of FeatureFlags mocks in com.android.systemui.statusbar.notification tests to FakeFeatureFlags
* Changes NotificationTestHelper to use constructor-provided FakeFeatureFlags; removes setter

Fixes: 290365128
Test: atest SystemUITests
Change-Id: Id16e9f59b800601dc04892477ede748c6ea2a8d3