Particles: Merge spawn & update into one code string with two functions

Change-Id: If57fb79db8f8c5fd185fefaa202167c8082dd846
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/229921
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
10 files changed