Previously, the transport registration extra was a private
constant. Since GMSCore depends on this value being passed, moving
it to a public API prevents having to define it twice in
framework and GMSCore, and ensures compatibility between the two.
TODO: Update GMSCore with this constant once this drops
into GMSCore.
Bug: 72730566
Test: 1) m -j ROBOTEST_FILTER=TransportManagerTest RunFrameworksServicesRoboTests
2) m -j ROBOTEST_FILTER=TransportClientManagerTest RunFrameworksServicesRoboTests
Change-Id: I8f7a2ca0275047a5d3cc1a530cd86499d0170f2f
1b9877ab44