Enable support for groups in External contacts accounts

This permits use with DavDroid and other external providers, who are unfairly
disadvantaged by Google, most likely to try to favour their own closed contact
syncing services. This levels the playing field, and lets external contacts
providers also do groups (as there's nothing otherwise stopping them)

Change-Id: I6b8133c03a1a647e52b32007a4891051c2c57999

src/com/android/contacts/common/model/account/ExternalAccountType.java

@@ -222,6 +222,11 @@ public boolean areContactsWritable() {
         return mHasEditSchema;
     }
 
+    @Override
+    public boolean isGroupMembershipEditable() {
+        return true;
+    }
+
     /**
      * Whether this account type has the android.provider.CONTACTS_STRUCTURE metadata xml.
      */