Change default ZenPolicy to use device defaults.

If a rule has no specified zen policy, it will apply the device default config rather than the policy attached to manual DND. Manual DND continues to have its configuration tied to the "global" ZenModeConfig. Further, as part of this change, all policies become fully specified: rather than having the default inherently implied by a null ZenPolicy, all rules have a fully-specified ZenPolicy object with all fields set.

This adds some upgrade-specific behavior for devices moving from a previous XML version, as all pre-existing rules get a copy of the manual/global DND policy to preserve consistency with their previous behavior.

Bug: 312739308
Test: manual by navigating Settings on device and observing changed upgrade behavior
Test: ZenModeHelperTest, NotificationManagerServiceTest, NotificationManagerZenTest (cts), ZenPolicyTest

Change-Id: I9130f5fa83f9dc8cd61dc576edf9cc65915d60be