[skottie] Pull SkottieProperty.h from Skotti.h

Forward declaration for PropertyObserver should work, but some G3
builds/configs are barfing...

TBR=

Change-Id: I47fc8d24d4e706df470c010c8fce13f07d726fd8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/293340
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
1 file changed