Temp disable Tethering if DUN is required

Getting source-based routing working is too risk for this point
in the project but tethering is broken otherwise, so disable
the tethering option if DUN is required until we can get a real
fix in.

bug:5495862
Change-Id: I5e852bf30c887599024a8b61af86ffec1d5333af

Author: Robert Greenwalt

services/java/com/android/server/ConnectivityService.java

@@ -2450,6 +2450,12 @@ public boolean isTetheringSupported() {
         int defaultVal = (SystemProperties.get("ro.tether.denied").equals("true") ? 0 : 1);
         boolean tetherEnabledInSettings = (Settings.Secure.getInt(mContext.getContentResolver(),
                 Settings.Secure.TETHER_SUPPORTED, defaultVal) != 0);
+        // Short term disabling of Tethering if DUN is required.
+        // TODO - fix multi-connection tethering using policy-base routing
+        int[] upstreamConnTypes = mTethering.getUpstreamIfaceTypes();
+        for (int i : upstreamConnTypes) {
+            if (i == ConnectivityManager.TYPE_MOBILE_DUN) return false;
+        }
         return tetherEnabledInSettings && mTetheringConfigValid;
     }