Refactor the platform_message_handler_ios:

1. Move @FlutterTaskQueue to header
2. Change the constructor only requires the platform task runner instead of the task_runners. Making this change so that we can use PlatformMessageHandlerIos while migrating to the embedder api. The embedder won't have access to the task_runners when using embedder api, but will have access to the platform task runner.
3. Minor: move impelmentation of DoesHandlePlatformMessageOnPlatformThread to the .mm file.

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