Support in ShadowResources for themes that use styles that are direct resource references

This fixes issue #649. That code exposed the following use case:

When using a Spinner with `spinnerMode=dropdown`, the constructor has a hardcoded (internal) default style (`spinnerStyle`).  See `Spinner.java` in the android source code.

The base theme in res/values/themes.xml in AOSP has the line:

```xml
<item name="spinnerStyle">@android:style/Widget.Spinner</item>
```

It was when trying to apply the default style using this default theme that Robolectric would blow up.