Make Swift plugin template swift-format compliant (#40195)

swift-format will alphabetize import statements, so Flutter* should be
before Foundation. Also, separating the "Generated file" comment
prevents the formatter from thinking that the comment is associated with
the following import, rather than the file, if it does re-order.

packages/flutter_tools/lib/src/plugins.dart

@@ -349,8 +349,9 @@ const String _objcPluginRegistryImplementationTemplate = '''//
 const String _swiftPluginRegistryTemplate = '''//
 //  Generated file. Do not edit.
 //
-import Foundation
+
 import {{framework}}
+import Foundation
 
 {{#plugins}}
 import {{name}}