Bluetooth: HFP_AG: Hold incoming call

Add a flag `BT_HFP_AG_INCOMING_HELD` to identify the incoming call is
held.

Add a function `bt_hfp_ag_hold_incoming` to put the incoming call on
hold.

Add a callback `incoming_held` to notify the application the incoming
is held.

Handle AT command `AT+BTRH=x` to hold/accept/reject the incoming call.

Improve function `bt_hfp_hf_accept` to accept the held incoming call.

Improve function `bt_hfp_hf_reject` to reject the held incoming call.

Improve function `bt_hfp_hf_terminate` to terminate the held incoming
call.

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