Add support of IActvityExtension and autoCreateExtensions to GameActivity

- all android extensions should implement IActivityExtension interface
- autoCreateExtensions is a list of autoCreates classname from extensions
  include.xml
Use extension initialize method for create auto-created extensions
Merge branch 'master' of https://github.com/openfl/openfl-native
Merge pull request #63 from sergey-miryanov/master

Add IActivityExtension and ability to auto-create android extensions