Make MainThreadUnfoldTransitionProgressProvider callback registration thread safe

This fixes a potential crash from UnfoldTransitionRepositoryImpl. When the transitionStatus flow was being collected from the background, it could have called the addCallback method from that bg thread. The "assertMainThread" is not needed anymore now.

Flag: ACONFIG unfold_animation_background_progress TRUNKFOOD
Test: MainThreadUnfoldTransitionProgressProviderTest
Fixes: 318958512
Change-Id: I41cb9ec5612a931b804f703993c324a2a2512575