Remove MutableStateFlow for transition current info

When multiple threads are attempting to begin transitions,
the MutableStateFlow can get stuck on a background thread
while emitting the update the consumers. This leaves a
small chance for other transitions to jump the queue, ones
that are scheduled onto a more favorable thread.

Avoid MutableStateFlow completely. There's no need, just
use a volatile to store state.

Fixes: 358533338
Test: atest KeyguardTransitionRepositoryTest
Flag: com.android.systemui.transition_race_condition
Change-Id: I1daf9d5074c966445a1ed54acc10fb91193d6c85