Bluetooth: Adding Sniff feature for PAN Profile

Sniff feature for PAN profile was not implemented in power
manager module of the stack, which was causing higher power
consumption after connecting PAN profile, Even though PAN
ACL link is idle & there is no data communication between
DUT & remote device. Adding sniff feature for PANU role &
NAP role to reduce power consumption.

Change-Id: Idf568f53a317abd536edf34fbdf1733fdf53b7d0