NEON optimizations for gray -> RGBA (or BGRA) conversions

Swizzle Bench Runtime
Nexus 6P 0.32x
Nexus 9  0.89x

PNG Decode Time (for test set of gray encoded PNGs)
Nexus 6P 0.88x
Nexus 9  0.91x

BUG=skia:4767
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1656383002
CQ_EXTRA_TRYBOTS=client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot

Review URL: https://codereview.chromium.org/1656383002
7 files changed