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!