Remove weak references in MR2ServiceImpl

The MR2ServiceImpl class is a singleton, and its lifecycle is coupled to
MediaRouterService, who is also in turn a singleton whose lifecycle is
coupled to system_server's lifecycle. So, there's no value in using weak
refs for those classes, as they should never be garbage-collected.

This change gets rid of weakrefs and makes cascade simplifications
without making further invasive changes.

Fixes: b/404574505
Test: atest CtsMediaRouterTestCases CtsMediaHostTestCases
Flag: EXEMPT I don't see how we can flag this change without massive churn.
Change-Id: Ib0dbb04b0540fe4497dc2417872d29f5d9369f93