Initial AR30ToARGB function to allow converion
from AR30 to other formats if necessary and/or
for testing.
Not optimized at this point.

Bug: libyuv:751
Test: LibYUVConvertTest.AR30ToARGB_Opt
Change-Id: I38ef192315240f3caa7aee0218b38d5e88a2849f
Reviewed-on: https://chromium-review.googlesource.com/833025
Commit-Queue: Frank Barchard <fbarchard@chromium.org>
Reviewed-by: richard winterton <rrwinterton@gmail.com>