Consolidate startedKeyguardTransitionStep and make it a StateFlow

I'm reducing the API surface of KTI such that there are fewer
non-flexiglass aware APIs that can eventually be removed or at least
deprecated. Also making this a stateFlow as we are trying to migrate
everything to stateFlows eventually.

Bug: b/330311871
Flag: EXEMPT refactor
Test: REFACTOR_ONLY
Change-Id: I5087008afa407fd9f775007a93a82b40eabd780f