We don't currently implement encoding for the duplex channel
for these codecs, so they are not fully functional as A2DP sinks,
and their main use is anyway with headphones.
Also, the number of endpoints in BlueZ appears to be limited, and
appears to be counted across all adapters. Unclear whether this comes
from AVDTP limitation, but currently plugging in a second BT adapter
causes the second media application registration to fail. This change
reduces the number of endpoints enough so that registration succeeds for
two adapters.
There doesn't seem to be a way to control the A2DP duplex microphone
HW volume gain, and devices sometimes have very low mic volumes.
Work around this by boosting the software volume scale by +20 dB. If it
causes clipping, the user can just reduce the volume to bring SW gain
below 1.0.
