Bluetooth: HFP_HF: Initiate SLC establishment

Add function `bt_hfp_hf_connect` to initialize the Service Level
Connection establishment procedure.

Add function `bt_hfp_hf_disconnect` to release the Service Level
Connection.

Clear HF object if the RFCOMM is disconnected.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>