Only offer migration between private volumes.

Without this change, there is a confusing option to migrate a public
SD card volume to internal storage, which will always fail if the
user picks it, since it's not supported under the hood.

Bug: 22966671
Change-Id: Id7cad07eea6583aace398a6d64383bf7d884ae9e

src/com/android/settings/deviceinfo/PrivateVolumeSettings.java

@@ -354,7 +354,9 @@ public class PrivateVolumeSettings extends SettingsPreferenceFragment {
         // Only offer to migrate when not current storage
         final VolumeInfo privateVol = getActivity().getPackageManager()
                 .getPrimaryStorageCurrentVolume();
-        migrate.setVisible(!Objects.equals(mVolume, privateVol));
+        migrate.setVisible((privateVol != null)
+                && (privateVol.getType() == VolumeInfo.TYPE_PRIVATE)
+                && !Objects.equals(mVolume, privateVol));
     }
 
     @Override