Particles: Do a cleanup/standardization pass on the API

For clients not using JSON, the factories were inconsistent.

Change-Id: Ifd920fa1e18f5edffa12de238af8488406951e5b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/257683
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
4 files changed