[Android] Use XWalkBaseActivity for each demo in Crosswalk Sample

1. XWalkBaseActivity will control the XWalkView lifecycle, such as onHide, onShow and onDestroy.
   All other demos need to override from this base activity.
2. Add CrosswalkSample to xwalk_builder. (it's not the final solution, will track it with XWALK-1593)

TODO: Removed alert in manifest demo, it will cause crash when onDestroy() is called.
      XWALK-1639

BUG=XWALK-1631
Merge pull request #1902 from kurli/sample-base-activity

[Android] Use XWalkBaseActivity for each demo in Crosswalk Sample