[SB Refactor] Cleanups in the carrier merged connection repo.

I verified that all the hard-coded values match the old pipeline
implementation, so removed that TODO and made one source of truth for
roaming.

Bug: 264684296
Bug: 238425913
Test: atest CarrierMergedConnectionRepositoryTest
Change-Id: Ia7fdd0c89f947544b9e53e01ca6558bc282b89d6