Track TX empty event to manage sniff timer

Use case:
OPP TX while inquiry is in progress.

Steps:
1. Send any file from DUT to remote using BT.
2. Accept file on remote.
3. During file transfer, start inquiry on DUT and
   observe behaviour.

Failure:
DUT sends sniff command in the middle of the transfer
though it is not required. eventually OPP Tx over
L2CAP file transfer fail.

Root Cause:
As per the current implementation, for tx it will
immediately starts idle timer without checking
for the completion status of tx.

Fix:
Handled the tx complete event from L2CAP properly
to start the idle timer to manage the sniff properly.

Change-Id: I298075590042e82a2f33837f6df6af0b2fd8179a