[Android] Hook the native destroying of XWalkview into GC

Now we need to restrict users to explictly call the onDestroy method
of XWalkView to make sure the native part of XWalkView will be
destroyed. The patch leverages the solution in Chromium and remove
the restriction.

BUG=XWALK-2010
Merge pull request #2137 from xingnan/2010

[Android] Hook the native destroying of XWalkview into GC