As the Visual Studio C++ compiler moves toward increasingly standards-compliant behavior, things it allows may become errors in the future (as seen in https://github.com/flutter/flutter/issues/163296 and the issues it references). To avoid a whole class of future problems, this enables `/permissive-` for the example apps of all of our Windows plugins that have native code, and fixes the resulting violations.

Fixes https://github.com/flutter/flutter/issues/163296