Change VNDK snapshot build variant to 'user'

Additionally build for all four archs separately:
(arch, variant) =
(arm64, armv8-a)
(arm, armv7-a-neon)
(x86_64, x86_64)
(x86, x86)

Test: development/vndk/snapshot/build.sh
Bug: 66059995
Change-Id: Ia31bc260d18e09a9499708c777e2374431470b4e