AndroidAudio uses multiple SoundPools to prevent heap size overflow

I've requested this improvement here: http://www.badlogicgames.com/forum/viewtopic.php?f=11&t=9316&p=42143&hilit=soundpool#p42143
But I choose to start with it.

The changes I made:
- Remove the SoundPool from AndroidAudio and place it in SoundPoolContainer.
- A private class SoundPoolContainer which holds the SoundPool and checks whether the pool is full.
- When a pool is full it creates a new SoundPoolContainer, this will prevent heap size overflow.
- Currently the max stream is set to 15. You'll get about 15 x ~50 KB sound. You'll have enough space left.

- I can't not check the size of the sound file, that is way the counter is used to track the amount of sounds that is loaded in the SoundPool.

Change the naming if it doesn't suits you.

(It's my first pull request using GitHub, don't shout at me if I'm doing something wrong)