AudioService: ignore restrictions on removed users.

During test suites we've observed that the USER_BACKGROUND broadcast
can be delivered after a user has been deleted, which can cause
operations like setUserRestriction() to fail.

Ignore these cases to avoid a runtime restart.

Bug: 255835206
Test: atest FrameworksMockingServicesTests:BroadcastRecordTest
Test: atest FrameworksMockingServicesTests:BroadcastQueueTest
Test: atest FrameworksMockingServicesTests:BroadcastQueueModernImplTest
Change-Id: Ia31f276079583c447f34815d0e0067959ea77668