Disable PiP button when playing audio-only assets

[theodab]

When playing an audio-only asset, the Picture-in-Picture button on the UI shows up. Pressing the button, however, results in an error.
If the button doesn't work, we should just not show it.

Similarly, there are issues if you enter PiP mode while playing a valid asset, and then load an audio-only asset. The PiP window will stay open, and the player will continue to be greyed out, but the audio-only asset will be playing, and the PiP window and the player will no longer be presenting the same asset. We should probably leave PiP mode automatically, in that case.