virtual_keys: redesign based on Android L

Android L soft keys are sexy!

 - xxhdpi extracted from Nexus 5 Android L Developer Preview's SystemUI.apk
 - CW, CCW rotate "back" button and adjust shadows
 - "home" button now as "enter" button
 - divide width by 4 and adjust all buttons to align horizontal location perfectly
 - remove blue bar just like Google's soft keys way
 - add a nice, black gradient
 - repeat those process with all resolutions
 - optimize final PNGs with optipng -o7 -zm1-9 -nb -nc -np

xxhdpi(Galaxy S4), xhdpi(Galaxy S3) in-action :
 - https://dl.dropboxusercontent.com/u/20724065/IMG_5136.JPG