BroadcastQueue: return reasons from skip policy.

Adjust BroadcastSkipPolicy to return the skip "reason" as a String to
let callers control the logging, but don't wire up into
setDeliveryState() just yet, since there's still a lot of duplicated
information, and some scripts may be parsing the current raw format.

Bug: 254521331
Test: atest FrameworksMockingServicesTests:BroadcastRecordTest
Test: atest FrameworksMockingServicesTests:BroadcastQueueTest
Test: atest FrameworksMockingServicesTests:BroadcastQueueModernImplTest
Change-Id: I1a74109e7188bd1e000639fc9924f538dceffc00