Add SnapshotPolicy argument to functions

[JakeWharton]

This controls whether or not the Composer/Recomposer setup also registers a global write observer that sends Snapshot apply notifications. If you have another Compose-based toolkit in your app (e.g., Compose UI) that is already sending these, there is no reason to run both.

Let's add this with #382

[JakeWharton]

Still going to move forward with this, but Google and JetBrains are jointly working on an extensions of the snapshot system for use outside Compose / Compose UI. So at some point this parameter might be completely obviated, but that's fine, and we'll deal with that when it happens.