Added fd for monitoring when the channel is not congested

Use case:
Skips and Media Audio Loss are Observed During PAN Connection

Steps:
1. Launched Local Music and Media Audio was in Headset.
2. Paired and Connected to other Phone to DUT Over BT Tethering.
3. While  Listening to Media Audio in DUT, Started Watching
   Youtube in other phone.
4. Observed Below Observations During Some 30mins of PAN Connection.

Failure:
Skips and Media Audio Loss are Observed During PAN Connection

Root Cause:
socket monitor thread keeps polling for PAN fd, eventually
it will make the BTU thread also busy, inturn NOCP processing
gets delayed.

Fix:
Add PAN fd for monitoring only when L2CAP channel related to
PAN profile is not congested. It will break loop by checking the
congestion status to put the fd for polling.

Change-Id: I2119c6c86d8e4798e516c512ad7aac7630b85a6a