[Android] Hotfix in Presentation API extension

This patch is to fix 2 issues:
o Place PresentationExtension into ..api.presentation package to follow
  the general pattern in Java. Otherwise, we will get error message when
  import xwalk_core_library into eclipse.
o Don't create PresentationExtension if failing to load the JS binding
  code from assets/

TODO:
1. Figure out why the external extension test crash when passing a empty string to
   create a new XWalkExtension.
2. Ensure the PresentationExtension code can also work on Android 4.0.