Align Post Init `registerPlugin` APIs across Platforms

Currently, Core Player supports the ability to add plugins to the instance after it is initialized. React/iOS/JVM/Android all have various levels of support for this and mixed APIs for this support. We should align this functionality across all platforms:

- React
 - Doesn't support adding Core plugins
- iOS 
 - Only supports adding Core plugins
- Android/JVM
 - Seemingly no support? 