[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
Merge pull request #2368 from gaochun/2385

[Android] Expose setNetworkAvailable method to XWalkView for CordovaWebView