Change the way ALARMS_ONLY/NO_INTERRUPTIONS rules block priority notifications

Instead of having ZenPolicy.Builder.disallowAllSounds() set priorityAllowed=none, we apply it directly when computing the consolidated policy from rules having one of these zen modes. Otherwise, it would also apply to rules with PRIORITY_ONLY filtering, which breaks existing use-cases.

The XML attribute where this value is stored was renamed so that we don't read previously-saved priorityChannels values. This is okay because the flag has not gotten past trunkfood and thus no rules should have a legitimate allowPriorityChannels=none value set (it being a new API), and neither should the manual rule's policy.

Test: ZenModeHelperTest (existing tests, added in ag/26227699)
Bug: 326306544
Change-Id: Ib1eb971f9bc605558d542909030da4d6eee6995a