Consolidate MediaRoute2Provider Callback dispatching

For route visibility, we want to be able to be able to examine routes
and sessions anytime a route provider's state or sessions change,
regardless of the type of provider. This CL will allow that by
mechanically consolidating the dispatching of the methods to the
registered MediaRoute2Provider Callback into the parent class.

Bug: 388071860
Test: N/A (no behavior change)
Flag: EXEMPT PURE_REFACTOR

Change-Id: Ibe46046ae7f1dc26ca4468d3e970aa843ecaf8f5