Bug 301813 Sunbird default theme's install.rdf should have appManaged="true"

patch by rob_strong@exchangecode.com r=shaver

calendar/sunbird/app/profile/extensions/{972ce4c6-7e08-4474-a285-3208198ce6fd}/install.rdf.in

@@ -21,6 +21,14 @@
     <em:name>Sunbird (default)</em:name>
     <em:description>The default theme</em:description>
 
+    <!-- EXTENSION AUTHORS!
+         DO NOT COPY THIS PROPERTY INTO YOUR INSTALL RDF FILES
+         It will cause users not to be informed of incompatibilities 
+         with your extension when they are updated with Software Update
+         and your extension will become unavailable to them!
+         -->
+    <em:appManaged>true</em:appManaged>
+
     <em:locked>true</em:locked>
 
     <!-- Front End Integration Hooks (used by Theme Manager)-->