addPlayer and playerUpdated effectively do the same, so merge the two implementations.
Also simplify nesting in setPlayerActive

Still behaves as before:

• It never switches away from a playing player when another starts playing
• It switches to a playing player if there was none before
• When current player gets paused, it switches to another playing one, if any, or a paused one, if any, or a stopped one, if any
• Closing a player, switches to another play one, if any, or a paused one, if any, or a stopped one, if any

Also makes it easier to adjust the code to support deduplication/proxying