[image_picker] Restructure Android ImagePickerDelegate codepaths

[stuartmorgan]

Originally the Android implementation had different Dart->Java entry points and different return types for picking one image vs picking multiple images. In flutter/packages#3476 I changed it so that the cross-language call is unified, and there's just a parameter for whether multi-selection should be allowed. The different intent codepaths, success helper methods, etc. are legacy at this point, and could likely be simplified to a single codepath with just the intent parameter being different.