Add support for rules with filter=ALL in ZenModesBackend

Add (hidden) getter and setter and new value for mAllowChannels, which is normally set through allowPriorityChannels().

Bug: 327419222
Bug: 331267485
Test: atest ZenModeTest
Flag: android.app.modes_ui
Change-Id: Iee75e4cdc2c89bbef60677ff5ee65690fcb29fc5