Add a wrapper module for depot_tools/osx_sdk: flutter_osx_sdk

~It looks like properties passed into `$depot_tools/osx_sdk` are not visible to
flutter_osx_sdk (see the examples/full.py file). So we need to do a s\$depot_tools/osx_sdk\$flutter/flutter_osx_sdk/
in the cfg files?~

Changed from inheritance to composition.
Changed the base class back to RecipeApi.

Change-Id: I1f83ab88fb913cc97f2ad24df57b81220e748e47
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/7000
Reviewed-by: Godofredo Contreras <godofredoc@google.com>
Commit-Queue: Weiyu Huang <weiyuhuang@google.com>