Change channels API to focus on priority channels.

This changes the ZenPolicy interface to also use booleans for setting channels, similar to how it was already implemented in NotificationManager.Policy.

Also renames the channels enum to be CHANNEL_POLICY instead of CHANNEL_TYPE, though the enum is also no longer surfaced in public API.

Bug: 308672314
Fixes: 315860311
Fixes: 315045597
Test: atest ZenModeHelperTest ZenPolicyTest ZenModeConfigTest NotificationManagerZenTest ZenModeFilteringTest

Change-Id: If19175a4aee030c51fbaa7dc3a1a83cae1759798