Unify Anime/MangaNetworkTask callback interface

While working on something different I noticed that the callback functions for AnimeNetworkTask and MangaNetworkTask always contain the same logic except for the result type (Anime/Manga). This means there is many duplicated code. I unified the callback to simplify this.
Now classes (ItemGridFragment and SearchActivity) have to implement only one interface with one callback function instead of two. Separation between Anime and Manga is possible by using the callback's _ListType_ parameter _type_.

I've tested this compared to 1.4 RC2 and noticed no differences, everything just works like before.