Add ResizeImage.policy (#121154)

* Add ResizeImage.policy

This adds a new `ResizeImage.policy` property that controls how `ResizeImage`
will interpret its `width` and `height` properties. The existing behavior is
preserved via `ResizeImagePolicy.exact` (default), but there is now the option
to use `ResizeImagePolicy.fit`, which satisfies the use case outlined in
flutter/flutter#118543.

The API doc assets were added in flutter/assets-for-api-docs#209

Fixes flutter/flutter#118543

* Docuemnt public member

* Remove protected annotation from overrides - was failing tests

* Fixed analysis of code in Dartdoc

* More dartdoc code analysis fixes

* One more fix

* Review comments

Author: tvolkert