GAP: Save pin code length for incoming legacy SAP connection

Use case: Carkit cannot reconnect to DUT in Sim Access Profile mode

Steps:
1. Pair with a car-kit (Agree all SAP, MAP, PBAP requests)
2. Initiate disconnection on DUT
3. Initiate re-connection on the car-kit.

Failure: SAP profile reconnection failing as dut gives negetive
linkkey reply.

Root Cause: sec_flags not getting updated without pin len and stack
triggering negative link key reply even linkkey is proper for 16 digit
authentication of SAP.

Fix: Save pin length during BTM_PINCodeReply.

Change-Id: I8300dd3ff497058291143313b169190910e0a7c5