Change BIND_APPWIDGET to be signatureOrSystem.

See http://b/issue?id=1839074 for more details.
diff --git a/core/res/AndroidManifest.xml b/core/res/AndroidManifest.xml
index 59b26a0..ca2db11 100644
--- a/core/res/AndroidManifest.xml
+++ b/core/res/AndroidManifest.xml
@@ -973,7 +973,7 @@
         android:permissionGroup="android.permission-group.PERSONAL_INFO"
         android:label="@string/permlab_bindGadget"
         android:description="@string/permdesc_bindGadget"
-        android:protectionLevel="signature" />
+        android:protectionLevel="signatureOrSystem" />
 
     <!-- Allows applications to change the background data setting
          @hide pending API council -->