[Sb refactor] Ensure we fetch default data subId on flow start

The old pipeline aggressively polls SubscriptionManager for the
`defaultDataSubId`. We don't want to do that since it's going to cause
IPC on the main thread, and it's not necessary.

This change enforces that we poll
SubscriptionManager.getDefaultDataSubscrpitionId() when the flow is
started, so that we always know the current value when collectors show
up. Further updates are still expected to come in through the broadcast.

Test: MobileConnectionsRepositoryTest
Bug: 270315194

Change-Id: I74aa6b9b456ca497b02de48d4c932a2b4814c97a