Apple Watch: app should copy and respect phone setting for whether to use cellular or not

If you turn on "Warn Before Using Data" the phone app will warn you before streaming or downloading episodes over your cellular connection. Presumably, if you turn that on, you care about your data and don't want it used unless you're explicit about it.

On the watch we seem to ignore this setting, it isn't copied to the watch and we have this logic in the `DownloadManager`:
```
#if os(watchOS)
let sessionToUse = WKExtension.shared().applicationState == .background ? cellularBackgroundSession : cellularForegroundSession
#else
let sessionToUse = useCellularSession ? cellularBackgroundSession : wifiOnlyBackgroundSession
#endif
```

While the other developers I've talked to aren't quite sure if this would apply when the watch sends things to the phone to be downloaded, it can't hurt to set this for what our users want. Eg: the "Warn Before Using Data" settings should be synced to the watch, and we should alter the logic so that cellular can't be used on the watch if that settings is true. If there are currently cases where Apple ignore this, it might be something they fix in future so it would be helpful if we did the right thing at our end.