Move Zen mode registerContentObserver calls to background handler.

Test: manual CPU profile
Bug: 324515627
Flag: ACONFIG com.android.systemui.register_zen_mode_content_observer_background DEVELOPMENT
Change-Id: Idec5d0f44030c5ea374b53d586cf01c67095ad7e
diff --git a/packages/SystemUI/aconfig/systemui.aconfig b/packages/SystemUI/aconfig/systemui.aconfig
index 7f229fb..f2a7105 100644
--- a/packages/SystemUI/aconfig/systemui.aconfig
+++ b/packages/SystemUI/aconfig/systemui.aconfig
@@ -469,3 +469,13 @@
         purpose: PURPOSE_BUGFIX
     }
 }
+
+flag {
+    name: "register_zen_mode_content_observer_background"
+    namespace: "systemui"
+    description: "Decide whether to register zen mode content observers in the background thread."
+    bug: "324515627"
+    metadata {
+        purpose: PURPOSE_BUGFIX
+    }
+}