Migrate summary text in wifi QR scan page to DescriptionMixin

The description in the setup flow would follow the new style to align to the left.

Bug: 188593594
Test: atest & manual
Change-Id: Idfc00f51710d725d5c653396902446f29861cb88

src/com/android/settings/wifi/dpp/WifiDppQrCodeScannerFragment.java

@@ -411,6 +411,7 @@ public class WifiDppQrCodeScannerFragment extends WifiDppQrCodeBaseFragment impl
     @Override
     public void onViewCreated(View view, Bundle savedInstanceState) {
         super.onViewCreated(view, savedInstanceState);
+        mSummary = view.findViewById(R.id.sud_layout_subtitle);
 
         mTextureView = view.findViewById(R.id.preview_view);
         mTextureView.setSurfaceTextureListener(this);