Use AlertDialog to alert Unsupported VPN type.

Follow up of aosp/2822270. Show an AlertDialog instead of a Toast
when starting an unsupported VPN type.

Test: Manual test
Change-Id: I2ffb1bd48dcb91624012c404b8a6eed1df9b0b8d

src/com/android/settings/vpn2/ConfigDialogFragment.java

@@ -209,7 +209,9 @@ public class ConfigDialogFragment extends InstrumentedDialogFragment implements
                 Toast.makeText(mContext, R.string.vpn_no_network, Toast.LENGTH_LONG).show();
             } catch (UnsupportedOperationException e) {
                 Log.e(TAG, "Attempted to start an unsupported VPN type.");
-                Toast.makeText(mContext, R.string.vpn_insecure_dialog_subtitle, Toast.LENGTH_LONG)
+                final AlertDialog dialog = new AlertDialog.Builder(mContext)
+                        .setMessage(R.string.vpn_insecure_dialog_subtitle)
+                        .setPositiveButton(android.R.string.ok, null)
                         .show();
             }
         }