Treat AHARDWAREBUFFER_FORMAT_R8_UNORM as kAlpha_8_SkAlphaType

I4846327b29736ee811672801a683deb287342a8e added
AHARDWAREBUFFER_FORMAT_R8_UNORM. Treat it as kAlpha_8_SkAlphaType, since
we'll be using it as an alpha mask.

Bug: 193170859
Test: TODO

Change-Id: I24bebaff63bb1e98ec8a8178797ec25d2f6bc89a
1 file changed