Fix compilation errors due to referencing the Android support library directly

This is to prevent compilation failures in projects that do not link the support-v4 library. The only other option I saw to fix this would have been to split up the android module into one for native fragments and one for support fragments, which I found would come with unjustified overhead. Hence I settled for the cheap way, which is to sacrifice type safety here.

Refs https://github.com/soundcloud/RxJava/issues/2

/cc @gbenson