Preload system routes on ProxyMediaRouter2 instances

Preloading system routes avoids an initial incongruent state where
MediaRouter2 does not store any routes, which happens between
instantiation and receiving the first batch of route updates from system
server.

Regular MediaRouter2 instances already preload system routes.

Test: atest CtsMediaBetterTogetherTestCases
Bug: 316584471
Change-Id: I62fd163108737a40bb2f82050e46d4c2c4c3f142