AudioSystem: refactor audio config cache and callbacks

Clean up implementation of audio configuration cache and
callback events from AudioFlinger:

- Define class AudioIoDescriptor for audio input and output
configurations outside of AudioSystem class.
- Do not use void * but an AudioIoDescriptor as argument to
audio config callbacks from AudioFlinger.
- Remove unused configuration events.
- Move AudioSystem audio input and output cache from static singletons to
members of AudioFlingerClient subclass.

Change-Id: I67c196c32c09ce2756af0755ee1fe631040c3270