Use the interface instead of the implementation for RF charging.

Should fix zerotheliger/PowerConverters#21
Tested all Redstone Arsenal tools, as well as items from
* IC2, Gregtech, & Gravisuite
* Applied energistics
* Thermal expansion
* Tinkers construct (with flux addition)
* Modular powersuits
All change when a vanilla chest is placed next to a universal charger with the items in it.