default playback speed: fix option and its UI

This actually implements the default speed option by setting a default value as needed by the iOS Settings UI which does not respect default values set in the Root.plist.

Additionally, this updates the player UI to respect this option as well by

1. showing the player's playback rate instead of a speed assumed by the UI
2. reset to the global setting's user-defined playback rate instead of reverting to hardcoded 1.0