Optimize and clean up ScanFilterQueue

- Improve encapsulation
- Reuse methods
- Remove unused `addServiceChanged()`
- Mark methods as @VisibleForTesting if only used for private and
  testing

Bug: 394958283
Test: atest ScanFilterQueueTest
Flag: Exempt refactor no-op
Change-Id: I54cd80f8577e6a50077415474a5961026567e2ce