Clean up nullability in MediaRouter2ServiceImpl

- As discussed offline, we agreed upon addiing @NonNull/@Nullable annotations to all arguments in public functions.
- For private functions we can also add @NonNull in cases where we actually check for nulls, (i.e cases where nulls break the execution).

Bug: 248537248
Test: atest MediaRouter2Test
Change-Id: I95a155fd762417bcfd8f2581ebe95f232ee6718e