[Pod] Move SettingObserver to a pod

Other settings classes are already in the util/settings pod, so move
SettingObserver to the same pod. This is required so that we can make
AirplaneModeRepository into a pod.

Bug: 407513670
Flag: EXEMPT refactor
Test: m SystemUI

Change-Id: I3654167effc97fe8345ea889551ebf07c684ae8b