Transition race condition fix part 2

Hold the mutex long enough to ensure the transition
starts/emits.

The theory is that unlocking the mutex early could transitions that
held the mutex to then resume, potentially reaching calls to start
prior to the first transition.

Fixes: 409585200
Test: atest KeyguardTransitionRepositoryImplTest
Flag: com.android.systemui.transition_race_condition_part2
Change-Id: If2089e66912d2f53bc14c8b2266b92d7e8d42445