[Android] Expose setNetworkAvailable method to XWalkView for CordovaWebView

To keep compatible with CordovaWebView, XWalkView.setNetworkAvailable
is required.

BUG=https://crosswalk-project.org/jira/browse/XWALK-2385
Yes if it's not in our Embedding API set now. Do you want to make a proposal for this API?