Make DrawableCrossFadeTransition#duration public

**Glide Version**: 4.15.1

**Use case background**:
I'm building a zoomable image viewer library that accepts a `RequestBuilder` so that developers can set parameters of an image using Glide's familiar APIs. For switching from a placeholder/thumbnail to a full-quality image, I'd like to cross-fade them by checking if a `DrawableCrossFadeTransition` is received in `Target#onResourceReady()`.

Can `DrawableCrossFadeTransition` expose its duration as a public property in some way? Coil does the same and it's very convenient.