[Android] Avoid the extension instance to be deleted twice

On android, the extension instances are owned by extension itself, while
XWalkExtensionServer is forced to maintain all instances created in xwalk
and the XWalkExtensionInstance is NOT ref-counted, as a result, the
instance would be deleted twice at exiting time and causes crash.

This fix is to address the above issue and ensure the extension instance
is only deleted by XWalkExtensionServer instead of extension itself on
Android.

BUG=https://crosswalk-project.org/jira/browse/XWALK-821