Revert^2 "Move SettingsProxyExt content observer registration call on bg thread."

This reverts commit bcf59292f8e8842d329c2f43451d875edd0f36c5.

Reason for revert: b/357575679 should never have been reverted
Change-Id: Ifd8a90e92389242fcf862716f3ddff2921229d6a
diff --git a/packages/SystemUI/aconfig/systemui.aconfig b/packages/SystemUI/aconfig/systemui.aconfig
index 201aaed..22bfe90 100644
--- a/packages/SystemUI/aconfig/systemui.aconfig
+++ b/packages/SystemUI/aconfig/systemui.aconfig
@@ -1304,4 +1304,14 @@
    namespace: "systemui"
    description: "Adding haptic component infrastructure to sliders in Compose."
    bug: "341968766"
+}
+
+flag {
+  namespace: "systemui"
+  name: "settings_ext_register_content_observer_on_bg_thread"
+  description: "Register content observer in callback flow APIs on background thread in SettingsProxyExt."
+  bug: "355389014"
+  metadata {
+    purpose: PURPOSE_BUGFIX
+  }
 }
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