Roll back to use bacground executor

Using dispatchers delays the registering of callback. This leads to some
issues with showing the seekbar on binding. Rolling back to executor
till we find out a solution.

Flag: ACONFIG com.android.systemui.media_controls_refactor DISABLED
Bug: 328207006
Test: atest SeekBarViewModelTest
Change-Id: I5678f4f9b821c80e12d4ec6274f32b913104f771