Make sure that invoking handlers is on main thread.

For new media architecture, we need to escape back to main dispatcher
before invoking handlers when registering callbacks.

Flag: ACONFIG com.android.systemui.media_controls_refactor DISABLED
Bug: 328207006
Test: Build.
Test: atest SeekBarViewModelTest
Change-Id: I2b7cf031007625b465d8d10b37e508946ae45ae6