Optimize and reduce size of PNG files

Done using:
find -type f -name *.png -exec optipng -o7 -nc -fix {} +
(-nc is required as TWRP png lib can't handle color type changes)
find -type f -name *.png -exec advdef -4 -z {} +
(use Zopfli compressor on insane prefix to reduce size even more)

Total amount of bytes saved: 317,491

Change-Id: I545c678f71942e6ceb95acb631841ea7ef82a049