WIP patch: test for get files with navigator.getDeviceStorage in apps/video/js/db.js 

On line 82 to 94 of gaia/apps/video/js/db.js
I added a snippet using navigator.getDeviceStorage('sdcard'); to get the video and subtitle files in sdcard on device.
But there is a problem that if files got successfully, program does not get into onsuccess function on line 88.