Support built in speaker on Wear

### What problem does this address?

What's the point of having an app to listen standalone on a watch if you can't listen standalone on the watch without an external speaker? Usually if you only have your watch and no phone, you likely aren't carrying your headphones with you either unless you explcitly planned for it (like a workout). Please add support for using the internal speaker.

### What is your proposed solution?

Allow users to select to use the internal speaker instead of being forced to pair a Bluetooth device.

### Did you search for existing feature suggestions?

- [X] I have searched for existing feature suggestions.

The issue names no files, tests, or entry points. Start by locating the Wear playback and audio-output implementation, then trace how Bluetooth output is enforced; done means users can select the watch's built-in speaker and standalone playback works without a paired external device.

Written by the indexing model from the issue text.