Add aconfig flag for wallpaper change receiver registration in bg.

This flag will be used to decide whether broadcast receiver registration
will happen on main thread or bg executor.

Test: manual flash and wallpaper change
Bug: 327315860
Flag: com.android.systemui.register_wallpaper_notifier_background
Change-Id: I149447a2df750740bd8f78777f4f51503f0a6019
diff --git a/packages/SystemUI/aconfig/systemui.aconfig b/packages/SystemUI/aconfig/systemui.aconfig
index 9fc3ca9..f3e2272 100644
--- a/packages/SystemUI/aconfig/systemui.aconfig
+++ b/packages/SystemUI/aconfig/systemui.aconfig
@@ -975,3 +975,13 @@
   description: "Shows a vertical bar at the right edge to indicate the user can swipe to open the glanceable hub"
   bug: "339667383"
 }
+
+flag {
+   name: "register_wallpaper_notifier_background"
+   namespace: "systemui"
+   description: "Decide whether to register wallpaper change broadcast receiver on background executor."
+   bug: "327315860"
+   metadata {
+     purpose: PURPOSE_BUGFIX
+   }
+}