Defer loading particle effect to load time.

Moving this out of the constructor means less going on at slide creation
time, making debugging easier.

Change-Id: I37bdd249abef663931bc8ef152a92a3a3436dcf4
Reviewed-on: https://skia-review.googlesource.com/c/192600
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>