Hearing Aid - codec switching

StartSendingAudio internally sets the codec to the HearingAidAudioSource.
OnDeviceAvailable callback triggers audio framework to  grab codec
configuration from HearingAidAudioSource. Those calls must be made in proper
order, otherwise audio framework will grab bad codec configuration.

This patch also remove default codec configuration, and crash if codec
configuration is not done correctly.

Test: use hearing aid supporting 24kHz codec
Change-Id: I9419d58fa8728fc8e0f0c2d2ed8d8d741309a9a3