Allow PlatformMacMain to be called multiple times

It is called each time the embedder initializes a Flutter view controller. The Mac shell (which does not have a view controller), calls it before NSApplicationMain.

Common items like the base::AtExitManager and the platform message loop are stored in `EmbedderState`

engine/src/flutter/BUILD.gn

@@ -7,8 +7,14 @@ group("default") {
   testonly = true
   deps = [
     "//sky",
-    "//sky/services/dynamic:sdk_lib",
   ]
+
+  if (is_ios) {
+    deps += [
+      "//sky/services/dynamic:sdk_lib",
+      "//sky/shell:flutter_framework",
+    ]
+  }
 }
 
 group("dist") {