bluez5: boost A2DP duplex microphone volume by 20 dB

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.