swap U and V and transpose conversion matrix, so I420AlphaToARGB and
I420AlphaToABGR share low level code.

Having less code with same performance allows more focused
optimization for future ARM versions.

R=harryjin@google.com
TBR=harryjin@chromium.org
BUG=libyuv:473,libyuv:516

Review URL: https://codereview.chromium.org/1422263002 .