Stop using (deprecated) SkGradientShader

- change GradientInfo to just store bool for premul
- Interpolation is now in SkGradient

Change-Id: Ie2784f78753ff92a30e43b99e0b653788bb8aa94
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1123156
Reviewed-by: Florin Malita <fmalita@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Mike Reed <mike@reedtribe.org>