Particles: Remove play-position thing, simplify mouse tracking

Change-Id: I36ce78ee1de8096d9aa164444e8af31b3831a3ab
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/258217
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
2 files changed