Fixes 2136: music keeps playing

A proposed fix for #2136 (with thanks to @HD_92 and @Tom-Ski) as discussed in http://www.badlogicgames.com/forum/viewtopic.php?f=11&t=16085#p69622 and following.

I've also added this to `didBecomeActive()` on the presumption that if `allowIpod` is `true`, then this is the behaviour that developers will be expecting. If it's not done here, then it becomes a subtle bug. However, if not desired, I'll add to the docs instead.

I've confirmed that this fixes #2136 on an iPad Mini running iOS 7.0.4 (and I also confirmed that the bug repros without the fix, just to double-check).

My Eclipse settings removed trailing spaces and unused imports. If this contravenes style guidelines, let me know and I'll disable and re-submit. Thanks!
If I'm understanding it correctly, developers can still override in their own didBecomActive() and set this back to true. Accordingly, my thinking was something like: "What's the best default here?". Given that at least 3 people have raised this as an issue, this seemed like a better default. However, in stating that, I realize that perhaps the other behaviour was working fine...  Unfortunately I don't know enough about how people use this on iOS to have a strong opinion either way. I would argue that this default is definitely a more consistent user experience. As it currently stands, sometimes they play music and go into the libGDX app and it keeps playing, but other times it does not. (I've had one customer support question already about this - and I don't have that many customers...).