Support (non-editable) display of DND with a filter != PRIORITY

Tweak ZenModesBackend to return the current DND interruption filter if manual DND is on. This will be used to show the corresponding policy (e.g. the policy for INTERRUPTION_FILTER_ALAMS) in this situation. The policy will be read-only in the UI since it cannot be customized.

Also clarify some related documentation.

Bug: 361586248
Test: atest ZenModeHelperTest ZenModeTest ZenModesBackendTest NotificationManagerZenTest
Flag: android.app.modes_ui
Change-Id: Ie09032bbf4ded76d02d6c9fcc47c746030536191