[Android] Notify XWalkClient instance when the page has started to load

When starting to navigate a page, we should notify XWalkClient that the page
has started to load, and give it a chance to call its onPageStarted
callback function. This follows the way Chromium AwContentsClient does.

BUG=https://github.com/crosswalk-project/crosswalk/issues/851
Merge pull request #855 from hmin/onpagestarted

[Android] Post a message when the page is starting to load