Adding ability to select recording and playback devices

[dturner]

Full changelog:

• Add the ability to select the recording and playback audio device. For example, you can now specify whether you want audio played through the built-in speaker, headphones or an attached USB audio device
• Refactored the UI into a number of common classes and moved non-UI code out of MainActivity
• Handle stream disconnects
• Refactored the StreamBuilder class into 2 separate *Engine classes. I did this because the point of the samples is to show the AAudio_* methods and hopefully this is a bit clearer now.
• Updated the README and screenshots