[Android] Implement setNetworkAvailable method in XWalkView

The setNetworkAvailable method allows XWalkView embedder to trigger
window.online/offline event and set navigator.onLine property on JS side when
the network connection is up or down. This fix is to implement this method by
adding a render process observer on native side to watch the network availability
change notification from browser process. At present, it is only used by Android
port.

BUG=https://github.com/crosswalk-project/crosswalk/issues/880
TEST=org.xwalk.core.xwview.test.SetNetworkAvailableTest