Register QS content observers on bg thread

Content observers do Binder calls to register/unregister, use the new async APIs to handle the registration.

Bug: 351766769
Test: created SettingObserverTest
Flag: com.android.systemui.qs_register_setting_observer_on_bg_thread
Change-Id: I3c25365ce7460f1d763223c8a7cc281a2f1be7a7
3 files changed