[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
you need to add `@XWalkAPI` to make this API available in `XWalkView` and `@hide` is still needed as it's not in Embedding API set.