Add feature flag REFACTOR_GETCURRENTUSER

Because in previous refactors some references from UserTracker have been
refactored to use SelectedUserInteractor as well, they need to bypass
the newly added flag until it is removed (and all references consume
the same source of truth).

Test: None
Bug: b/303808405
Bug: b/297839820
Flag: b/305984787
Change-Id: I8ec2e8af18b652c32daf9cd9eafeae9a94ec6dce