Some A2DP codecs (like FastStream or aptX Low Latency) are bi-directional
and can be used for both music playback and audio calls. This patch
implements usage of backchannel if A2DP codec provided by pulseaudio API
supports it.

A2DP codec switching needs new version of bluez as older version does not
provide needed org.freedesktop.DBus.ObjectManager and RegisterApplication
DBus APIs.

Pulseaudio use for each A2DP codec separate pulseaudio profile, therefore
codec switching is implemented via changing pulseaudio profile and
currently used A2DP codec is visible in pulseaudio profile.

Getting list of supported codecs by remote device is supported only by new
version of bluez daemon.

If old bluez version is detected then only one SBC codec is supported.