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
diff --git a/packages/SystemUI/aconfig/systemui.aconfig b/packages/SystemUI/aconfig/systemui.aconfig
index e2ecda3..63ce7eb 100644
--- a/packages/SystemUI/aconfig/systemui.aconfig
+++ b/packages/SystemUI/aconfig/systemui.aconfig
@@ -1143,3 +1143,13 @@
        purpose: PURPOSE_BUGFIX
    }
 }
+
+flag {
+  namespace: "systemui"
+  name: "qs_register_setting_observer_on_bg_thread"
+  description: "Registers Quick Settings content providers on background thread"
+  bug: "351766769"
+  metadata {
+    purpose: PURPOSE_BUGFIX
+  }
+}