Add Android platform support (via Skip) for OpenCombineShim

[tifroz]

Android (via Skip) always requires OpenCombine since there's no native Combine. The traits mechanism doesn't work in Skip's build pipeline, so Android needs to be added without the trait condition.

As I understand it, Linux behavior is functionally unchanged since the trait is auto-enabled by default on non-Darwin platforms anyway (see preprocessor condition below, found at the bottom of Package.swift)

#if !canImport(Darwin)
  package.traits.insert(
    .default(enabledTraits: ["OpenCombineSchedulers"])
  )
#endif