Extract a shared MediaQueryData for padding and viewInsets (#13434)
Extract a shared MediaQueryData for padding and viewInsets (#13434)

Also adds a TODO that should have landed with flutter/flutter#13423.
tvolkert/flutter,cbracken/flutter,flutter/flutter,cbracken/flutter,Hixie/flutter,flutter/flutter,flutter/flutter,tvolkert/flutter,Hixie/flutter,Hixie/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter,Hixie/flutter,Hixie/flutter,tvolkert/flutter,tvolkert/flutter,tvolkert/flutter,flutter/flutter,Hixie/flutter,tvolkert/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter,cbracken/flutter,cbracken/flutter,cbracken/flutter,cbracken/flutter,flutter/flutter,Hixie/flutter,cbracken/flutter,flutter/flutter,Hixie/flutter,cbracken/flutter,flutter/flutter,tvolkert/flutter,flutter/flutter,Hixie/flutter