Fix saving Policy with STATE_PRIORITY_CHANNELS_BLOCKED

The state field was always dropped in NM.setNotificationPolicy(). This was okay when the only bit in the state was STATE_CHANNELS_BYPASSING_DND, which cannot be set from outside NMS. However, STATE_PRIORITY_CHANNELS_BLOCKED can be -- although, as a hidden API, only from some callers.

One subtlety is that we cannot just forward the state verbatim from the supplied Policy, because the caller likely doesn't know the current value of STATE_CHANNELS_BYPASSING_DND (in fact, with the NM.Policy <-> ZenPolicy conversion that Settings does, this value is outright dropped). Since ZenModeHelper will take the state (unless it's STATE_UNSET), NMS must be careful to merge the modifiable bits from the new Policy with the unmodifiable bits from the current Policy before passing it on.

Fixes: 369721344
Test: atest NotificationManagerServiceTest
Flag: android.app.modes_ui
Change-Id: I71d81b32e4dda482f2e738582baba36a79efc19a