[Android] Use XWalkBaseActivity for each demo in Crosswalk Sample

XWalkBaseActivity will control the XWalkView lifecycle, such as onHide, onShow and onDestroy.
All other demos need to override from this base activity.

TODO: Removed alert in manifest demo, it will cause crash when onDestroy() is called.
            Will fix it in another PR.

BUG=XWALK-1631