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>

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index b4140a3..c2d04fe 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -2440,7 +2440,7 @@
   }
 }
 
-if (is_ios && skia_use_metal) {
+if (is_ios && skia_use_metal && !skia_enable_flutter_defines) {
   group("minimal_ios_mtl_skia_app") {
     deps = [
       "experimental/minimal_ios_mtl_skia_app",