Move transport constant to BackupTransport API

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