Simplify experimental pedometer example (#2104)

Since ffigen added support for
[`NativeCallable.listener`](https://api.flutter.dev/flutter/dart-ffi/NativeCallable/NativeCallable.listener.html)
to its ObjC bindings, this example can be simplified. We can replace the
`Dart_Port` logic with `ObjCBlock.listener`, which lets us get rid of
most of the native code.

We still need a small bit of native code to `retain` a reference to the
callback's arguments before invoking the listener, otherwise the
arguments may be ref counted and deleted before the Dart side of the
callback is invoked. See https://github.com/dart-lang/native/issues/835

experimental/pedometer/ffigen.yaml

@@ -7,17 +7,18 @@ compiler-opts:
   - "-F/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk/System/Library/Frameworks"
   - "-mios-version-min=13.0"
 exclude-all-by-default: true
+functions:
+  include:
+    - "wrapCallback"
 objc-interfaces:
   include:
     - "CMPedometer"
-    - "PedometerHelper"
     - "NSDate"
     - "NSDateFormatter"
 headers:
   entry-points:
-    - "/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk/System/Library/Frameworks/CoreMotion.framework/Headers/CMPedometer.h"
     - "src/pedometerHelper.h"
 
 # To use this API, you must include the NSMotionUsageDescription key in your app’s Info.plist file 
 # and provide a usage description string for this key. 
-# The usage description appears in the prompt that the user must accept the first time the system asks the user to access motion data for your app.
+# The usage description appears in the prompt that the user must accept the first time the system asks the user to access motion data for your app.