Double disable of sound production

[jbphet]

In soundManager.addSoundGenerator there is a bit of code that adds an enable control property to each sound generator that is added. It looks like this:

    // add the global enable Property to the list of Properties that enable this sound generator
    soundGenerator.addEnableControlProperty( this.enabledProperty );

If I remember correctly, we did this so that we wouldn't end up in a situation where the user enabled sound part-way through the production of a sound and just heard the tail end of it, or at least that we had full control over this situation. However, I believe that this is contributing to a problem where we hear some crackling when sound is disabled and then re-enabled because there are a bunch of sound generators that are being turned up at the same time as the master gain control. When I removed the above line of code, this problem seemed to go away. Note that I say "seemed to go away" because it's not particularly easy to reproduce, and I'd like to have QA test it before I'd be truly confident.

I think this is contributing to phetsims/faradays-law#192, and I feel like I need to talk this over with the rest of the sound design team before moving any further on it. If we do move forward, I feel like the first step would be for me to produce a dev version with this fix in place, have QA test it and verify that the behavior is better. If it is, I should submit the change permanently and then have QA test the existing sims that have sound and verify that nothing has broken as a result.