Make SliceViewManager calls from the main thread

SliceViewManager is not thread-safe which causes some
ConcurrentModificationException.

Flag: aconfig new_volume_panel NEXTFOOD
Test: atest AncSliceRepositoryTest
Test: manual on the phone with the headset. Check that callback register/unregister and slice binding happens on the main thread.
Fixes: 336713865
Fixes: 338240912
Change-Id: Idbc789d5b008fcb87551a95649f9eb4fc5fef582