[Android] React to activity state change within XWalkView

Previously, XWalkView relies on embedders to manage the
resource of XWalkView based on Activity lifecycle.

In this commit, implement XWalkView as ActivityStatusListener
and handle the lifecycle state change by itself.

BUG=
Yes, I see the comments for each methods have updated, the following paragraph for XWalkView class should also be updated accordingly:
```
 * <p>Unlike other Android views, this class has to listen to system events like application life
 * cycle, intents, and activity result. The web engine inside this view need to get and handle
 * them. And the onDestroy() method of XWalkView MUST be called explicitly when an XWalkView
 * won't be used anymore, otherwise it will cause the memory leak from the native side of the web
 * engine. It's similar to the 
 * <a href="http://developer.android.com/reference/android/webkit/WebView.html#destroy()">
 * destroy()</a> method of Android WebView. For example:</p>
```