Add threading annotations to MediaRouterService

- Added @GuardedBy annotation to state fields that require to be syncronized.
- Added @GuardedBy annotation to the methods with the suffix 'Locked', It is enough to make sure the method call is wrapped in syncronized block each time it's called, however this, I added the annotation to all methods with the suffix 'Locked' for convience.

Bug: 252765931
Test: atest MediaRouterTest
Change-Id: If31d84058d82a0f67b64f6eed7556ef36a58b51c