rename I400 to J400 and I400 reference to I400.  J400 is a simple replication of values to convert to RGB, which is what the old I400 was.  I400 reference is the Y part of the YUV formula, so renaming that to I400.
BUG=none
TESTED=libyuvTest (5925 ms total)
R=tpsiaki@google.com

Review URL: https://webrtc-codereview.appspot.com/50369005

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1333 16f28f9a-4ce2-e073-06de-1de4eb20be90