Make converter accept enum ordinals

Make `Converter` accept flag or enum ordinal values as well as their string values.

Closes #992. `PreferenceFragment` was not able to be inflated because in the default layout (`preference_list_fragment.xml`) the `scollbarStyle` attribute was [set with an integer](https://github.com/android/platform_frameworks_base/blob/dbccd44a638ae8705a5b14bff8b2dd74abc26045/core/res/res/layout/preference_list_fragment.xml#L34) rather than an enum value. Robolectric's `Converter` was then saying the value was not found.

Because the Android framework is obviously able to inflate this successfully, it seems that `Converter` should accept enum ordinals as well as their string values.