Partial revert of "[SB Refactor] Remove unused `isDefaultDataSubscription` flow."

Reference commit 3747a4723edec8d89c53c2ff52091680cf8f7bc6.

The reverted commit removed MobileConnectionsRepository.defaultDataSubId
and MobileConnectionRepository.isDefaultDataSubscription, the latter of
which was truly unused and incorrectly defined. The former, however, is
the one needed for NOT_DEFAULT_DATA

Test: MobileConnectionsRepositoryTest
Bug: 238425913
Change-Id: I0945a29d50e42ae8f0a09fd54fcfc5d4b4e4affe
6 files changed