Remove lousy synchronization

An @AnyThread method was being used for syncrhonizing user switching
states without having a synchronization block. We were also relying
on callbacks that might be unordered, especially during user switching.

Fixes: 131914931
Test: switch between users that have different dark mode settings
Change-Id: Ie85c2a2a087edcd35c825caf8193aeabb990ea6c