Compose UI Box stretch only partially working

[JakeWharton]

From @colinrtwhite in #1854:

This PR avoids adding support for the case where CrossAxisAlignment.Stretch and Constraint.Wrap are applied along the same axis. This is a bit tough to implement in Compose UI since we can only call Measureable.measure at most once, which means measuring the child locks its size and it can't be expanded to fill the size of the largest item in the box.

I think we might be able to implement this with a combination of Measurable.[min/max]Intrinsic[Width/Height], but I'll tackle that in a separate PR.