Add support for rules with filter=ALL in ZenModesBackend

This is represented as an extra, special value for the AllowChannels field in ZenPolicy, with special treatment.

Bug: 327419222
Bug: 331267485
Test: atest ZenModeTest
Flag: android.app.modes_ui
Change-Id: I44440e40e729d295150b36beda824ddbfa1f170d