[skottie_viewer] Remove unnecessary Scenic deps.

Needed to prevent symbol collisions with internal
Scenic implementation (specifically, symbols that
will be used by Escher).

Add fostr dependency due to SCN-1193.

TEST=scenic_tests pass in CQ

Change-Id: Ied4058580aab543c051d91147d5da398b48f4833
diff --git a/bin/ui/skottie_viewer/BUILD.gn b/bin/ui/skottie_viewer/BUILD.gn
index 14b77a7..ca7eb4a 100644
--- a/bin/ui/skottie_viewer/BUILD.gn
+++ b/bin/ui/skottie_viewer/BUILD.gn
@@ -29,9 +29,10 @@
 
   deps = [
     ":skottie_loader",
-    "//garnet/lib/ui/gfx",
     "//garnet/public/fidl/fuchsia.ui.app",
     "//garnet/public/fidl/fuchsia.ui.viewsv1",
+    # TODO(SCN-1193): Remove fostr dependency.
+    "//garnet/public/lib/fostr/fidl/fuchsia.ui.scenic",
     "//garnet/public/lib/component/cpp",
     "//garnet/public/lib/fidl/cpp",
     "//garnet/public/lib/fsl",