Migrate testing_app to UIScene lifecycle (#2836)

## Summary
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- Migrate `testing_app` iOS project to the UIScene lifecycle as per the
[migration
guide](https://docs.flutter.dev/release/breaking-changes/uiscenedelegate)
- Update `AppDelegate.swift` to use `@main`,
`FlutterImplicitEngineDelegate`, and move plugin registration to
`didInitializeImplicitFlutterEngine`
- Add `UIApplicationSceneManifest` to `Info.plist`

Part of flutter/flutter#176957

## Test plan
- [x] Ran `testing_app` on iOS simulator — app launches and works
correctly

---------

Co-authored-by: Eric Windmill <eric@ericwindmill.com>

pedometer/lib/pedometer_bindings_generated.dart

@@ -45993,9 +45993,7 @@ class ObjCBlock_ffiVoid_ObjCObject_ffiUnsignedLong_bool extends _ObjCBlockBase {
                       ffi.UnsignedLong,
                       ffi.Pointer<ffi.Bool>,
                     )
-                  >(
-                    _ObjCBlock_ffiVoid_ObjCObject_ffiUnsignedLong_bool_closureTrampoline,
-                  )
+                  >(_ObjCBlock_ffiVoid_ObjCObject_ffiUnsignedLong_bool_closureTrampoline)
                   .cast(),
           _ObjCBlock_ffiVoid_ObjCObject_ffiUnsignedLong_bool_registerClosure(
             (