Replace MediaRouter2ManagerTest's assertions with more advanced techniques

- added BundleSubject
- assertThat(size).isEqualTo(...) is replaced with hasSize
- improved arrays assertion with assertThat(...).contains(...) and
  assertThat(...).doesNotContain(...)

Bug: b/244428131
Test: atest MediaRouter2ManagerTest
Change-Id: Ica1f94a3d08baa0c9b115f40f89f190849886de5