Request focus on XWalkView to listener volume button

The XWalkView has no focus after loading web page, it can't receive the
onKeyListener and onKeyDown event, so the volume-button plugin and the
Cordova volume event can't work.

BUG=XWALK-2424 XWALK-2835
Merge pull request #150 from fujunwei/volume-button

Request focus on XWalkView to listener volume button