[Android] Increase Embedding API version to 2.1

2.0 => 2.1 contains the following API changes:

XWalkResourceClient:
 + public boolean shouldOverrideUrlLoading(XWalkView view, String url);

XWalkUIClient:
 + public void onReceivedTitle(XWalkView view, String title);
 + public boolean shouldOverrideKeyEvent(XWalkView view, KeyEvent event);
 + public void onUnhandledKeyEvent(XWalkView view, KeyEvent event);
 + public void onPageLoadStarted(XWalkView view, String url);
 + enum LoadStatus;
 + public void onPageLoadStopped(XWalkView view, String url, LoadStatus status);

+ XWalkExtension:
 + public XWalkExtension(String extensionName, String sourceContent);
 + public XWalkExtension(String extensionName, String sourceContent, String[] entryPoints);
 + public final void postMessage(int instanceID, String message);
 + public final void broadcastMessage(String message);
 + public abstract void onMessage(int instanceID, String message);
 + public abstract String onSyncMessage(int instanceID, String message);

BUG=https://crosswalk-project.org/jira/browse/XWALK-2068