Fix race condition in transition from dream -> hub

We should only transition to the hub from the dream if we did not
already start another transition to somewhere else. This fixes the case
where we may transition to the hub after a transition to the bouncer has
already occurred.

Bug: 358695869
Test: atest FromDreamingTransitionInteractorTest
Flag: com.android.systemui.communal_hub
Change-Id: Iecc5d5119e166561a67bb46e7424a13d32873125