CastDetailsView: Add unit tests for view model and dialog presenter

Add CastDetailsViewModelTest and MediaRouteDialogPresenterTest for
better test coverage.

Bug: 378514236
Flag: com.android.systemui.qs_tile_detailed_view
Test: CastDetailsViewModelTest, MediaRouteDialogPresenterTest
Change-Id: I390a7e81ccc1e35ddc6287fc76aee887a9bf591d