Similar to other animatable text props, implement support for stroke
width.
Change-Id: Ia6ef65e8528edfec20def1b77da8df817e56cc7e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/546244
Reviewed-by: Jorge Betancourt <jmbetancourt@google.com>
Commit-Queue: Florin Malita <fmalita@google.com>
