Bluetooth: L2CAP: Introduce internal BR/EDR connection L2CAP notifier API

Adds dedicated function informing L2CAP layer about established BR/EDR
connection. Existing 'bt_l2cap_connected()' connection notifier is
refactored to use new API 'bt_l2cap_br_connected()' which is implemented
in l2cap_br.c file.
Currently the new handler is just stub since proper internals of
BR/EDR L2CAP signal channel management are not yet implemented
in l2cap_br.c file.

Change-Id: Idef9be63338c6bb4b3315ec0452d84f2431fbaa2
Signed-off-by: Arkadiusz Lichwa <arkadiusz.lichwa@tieto.com>