Changed mp3 players to significantly reduce micromanagement.

Made the mp3 player morale bonus decay significantly faster when no
longer listening and significantly reduced its battery consumption so
total morale duration remains similar. Fixes #5941

In short, it's now reasonable to use the mp3 player continuously
rather than to listen for one turn every 30 minutes. This costs about
twice as many charges as optimally micromanaging it used to. If you
still want to micromanage it you now have to listen one turn out of
20, at which point you may as well be scumming the charge calculation
itself.

Making something like "recently enjoyed music" into a disease might be
nice in the future, especially to unify the system with car stereos
and have finer control over decay and stacking than a generic morale
bonus, but for now this way involves minimal code changes.

Also:

* Made mp3 players no longer provide any morale while asleep, to
prevent easily stacking the high int "classical music" bonus.

* Reduced the song frequency from one per 10 turns to one per 5
minutes to keep things reasonable when used continuously.

## Detailed comparison:

### Old system:

1. Spend 5 charges to play for 50 turns. Morale +50.
2. Turn it off for 299 turns to save battery. Morale still +50.
3. Spend 0.1 charges to play for 1 turn, refreshing the duration.

Steps 2 and 3 then repeat forever until you go to sleep. For 16 hours
of wakefulness, it costs 5 charges to build up the bonus and 32
charges to maintain the bonus, for an optimal total of 37 charges per
day.

If you have to do a single task that takes longer than 30 minutes, it
takes up to 5 charges to build the bonus again. This is normally
pretty rare, and even then it's often with things where current morale
is less of a concern than the focus already gained from it having been
high.

The only realistic time it used more charges than that were when the
player didn't bother to micromanage their music consumption (to the
ludicrous extreme of listening to 6 seconds of music every half
hour). That's not difficulty, that's tedium.

### New system:

1. Spend 0.3 charges to play for 50 turns. Morale +50.
2. Spend 63.6 charges to leave it on for 9550 turns. Morale +50.

Granted, you could still micromanage the new system, but that would
mean turning it on for one turn out of every 20, and charge
consumption in general starts to break down at that level of
micromanagement anyway.