treewide: optimize PNGs with optipng

Command :
find . -name '*.png' -exec optipng -o7 -zm1-9 -nb -nc -np {} \;

This reduces size on all PNGs without any quality loss or format changes

This may help improve speed on loading images,
and ultimately make the final recovery.img smaller
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