[Android] Add sample and test cases for Embedding API

This patch is to add sample and test cases for the following interface of XWalkUIClient:
onCreateWindowRequested
onIconAvailable
onReceivedIcon
suggest rewirte as below to avoid indent
```
if (event.getAction() != KeyEvent.ACTION_DOWN || keyCode != KeyEvent.KEYCODE_BACK) {
    return super.onKeyDown(keyCode, event);
}

if (mXWalkViewHistory.getLast().getNavigationHistory().canGoBack()) {
...
```