audio policy: add support for external audio sources

Add support for activity on external audio sources.
An external source reflects activity on an input audio device
that must be controlled (both routing and volume) by the
audio policy manager.
First, the input device must be connected with setDeviceConnectionState().
Then, the source activity is indicated with startAudioSource() and
stopAudioSource() APIs.
startAudioSource() indicates the source device with an audio port configuration
and the use case by the audio attributes.

Once a source is active, its routing and volume are controlled by the policy manager
as it would for a software source (AudioTrack).

Change-Id: If5805d58a4356b2f681f1aabf54375f62b55b98a