[Android] Fix XWalk crash issue when call onDestroy.

The root cause is that destructor function was repeatedly executed.
In the destructor function of XWalkExtensionAndroid, call the
XWalkBrowserMainParts::UnregisterExtension, it erases the extension
and deletes it, then reentry the destructor function.

The fix is to move the unregister enxtension to DestroyExtension.

BUG=https://crosswalk-project.org/jira/browse/XWALK-352