Allow only one episode screen in the backstack at a time on the watch

In order to simplify navigation on the watch, it may be good to make it so that there is only a single episode screen in the backstack at a time instead of making it possible to have multiple different episodes in the backstack.

This was brought up [here](https://github.com/Automattic/pocket-casts-android/pull/908#issuecomment-1526017347).

Start by tracing the watch episode navigation and back-stack entry points, then read the discussion in pull request #908 linked from the issue. Done means navigating between episodes on the watch leaves no more than one episode screen in the backstack at a time.

Written by the indexing model from the issue text.