Ensure SafetySourceReceiver only runs for system user

This prevents a new phone process from being launched for every user (including guests)

Bug: 330095930
Change-Id: If49ebd97dff1844d0918587fa513473de296d23a
Test: manually: build, switch to guest user, assert no new phone process
diff --git a/AndroidManifest.xml b/AndroidManifest.xml
index 5c8504a..1190d38 100644
--- a/AndroidManifest.xml
+++ b/AndroidManifest.xml
@@ -596,7 +596,8 @@
 
         <receiver
             android:name=".security.SafetySourceReceiver"
-            android:exported="false">
+            android:exported="false"
+            androidprv:systemUserOnly="true">
             <intent-filter>
                 <action android:name="android.safetycenter.action.REFRESH_SAFETY_SOURCES"/>
             </intent-filter>