Don't accept a user-defined dun-capable connection

If we're asked to connect to a DUN APN and we have a carrier specified
DUN APN setting, verify what we're connected to is the same as what the
carrier specified before accepting it.

bug:4048013
Change-Id: I91edc4a1342cb40c1f6959e149303b7d76710f96

Author: Robert Greenwalt

telephony/java/com/android/internal/telephony/gsm/GsmDataConnectionTracker.java

@@ -361,6 +361,12 @@ public boolean isDataConnectionAsDesired() {
     @Override
     protected boolean isApnTypeActive(String type) {
         // TODO: support simultaneous with List instead
+        if (Phone.APN_TYPE_DUN.equals(type)) {
+            ApnSetting dunApn = fetchDunApn();
+            if (dunApn != null) {
+                return ((mActiveApn != null) && (dunApn.toString().equals(mActiveApn.toString())));
+            }
+        }
         return mActiveApn != null && mActiveApn.canHandleType(type);
     }