ABGR is the more common format on Android.
This CL converts 10 bit AR30, to standard 8 bit ABGR.
Unoptimized but allows better testing and feature completeness.

Bug: libyuv:751
Test: LibYUVConvertTest.AR30ToABGR_Opt
Change-Id: I0c7e7273158be215129e0a1d355587ae15942299
Reviewed-on: https://chromium-review.googlesource.com/891694Reviewed-by: Miguel Casas <mcasas@chromium.org>