Fix "unknown font id" issue

This should fix "unknown font id" bug reported [here] (http://stackoverflow.com/questions/18882435/robolectric-error-unknown-font-id).

The root cause is that both native Typeface class and ShadowTypeface have a cache of font instance, but only ShadowTypeface clear its cache by Robolectric.reset(), which causes a cache inconsistency. I don't think we need to clear cache in ShadowTypeface class by Robolectric.reset().