Bump ffigen from 14.0.1 to 15.0.0 in /experimental/pedometer (#2477)

Bumps [ffigen](https://github.com/dart-lang/native/tree/main/pkgs) from 14.0.1 to 15.0.0.
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experimental/pedometer/pubspec.yaml

@@ -14,7 +14,7 @@ dependencies:
   ffi: ^2.1.2
 
 dev_dependencies:
-  ffigen: ^14.0.0
+  ffigen: ^15.0.0
   jnigen: ^0.12.1
   flutter_test:
     sdk: flutter