Add factory methods for the system providers

This will allow the SystemMR2Provider2 to initialize part of the super
class state only after it has initialized its final fields. As part of
this change, we also move non-trivial initialization logic out of the
constructors.

Bug: b/362507305
Test: atest CtsMediaBetterTogetherTestCases CtsMediaHostTestCasts
Flag: com.android.media.flags.enable_mirroring_in_media_router_2
Change-Id: I6cb572410397ce4b4b8e27a77b1e5488e660b9cf