Particles: Store stable randoms after spawn affectors

This fixes a subtle bug with correlated behavior, where the spawn and
update affectors would get the same sequence of random values, leading
to confusing patterns.

Bug: skia:
Change-Id: Ieaace68bfe92281ec81497a8c6ce0108df996a4c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/199251
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>