Adding a method to share a screenshot with IntentChooser from Unity3D

This extends the base plugin with a ShareScreenShot method, allowing an image and string to be sent to an IntentChooser which is then displayed and interacted with on top of Unity. This uses a callback reference to directly call a function in C# when the IntentChooser is presented. It also uses a FileProvider to serve the saved image to the IntentChooser without requiring the external write permission, or those annoying permission popups that are now part of android. You can find the source code for this tutorial at: https://github.com/cwgtech/AndroidUnityShareScreenShot.git Follow me on Twitter @cwgtech, or view my programming blog at http://www.cwgtech.com