Don't build the minimal Metal app when building for Flutter.

Change-Id: I6bfdbcfbfd53b60f8d9f5fee3d2621c3f3e8c147
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/245476
Reviewed-by: Hal Canary <halcanary@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>