[skottie] Fix font fallback in viewer and skottie_tool

After https://skia-review.googlesource.com/c/skia/+/782356 we need an
explicit font manager for font fallback to work.

Change-Id: I04d67a0ec0caee18ac230fded199f237722ff6e6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/798676
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Jorge Betancourt <jmbetancourt@google.com>
3 files changed