Rename Alpha type constant

Rename PREMUL and PREMULT constants to PREMULTIPLIED and
UNPREMULTIPLIED to better match with Skia naming convention
for SkAlphaType.

Fixes: 269491516
Test: Re-ran MeshTest
Change-Id: If9d96143fdcb2e77803830f2be0c85160bec108d
2 files changed