Bluetooth: HFP_HF: Hold incoming call

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

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

Handle the response `+BTRH` to get the incoming call status.

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

Remove callback `call_setup`. Add callback `outgoing` for call_setup=1,
`incoming` for call_setup=2, and `remote_ringing` for call_setup=3.

Remove callback `call`. Add callback `accept`, `reject`, `terminate`,
and `incoming_held`.
`accept` is triggered if the call is active.
`reject` is triggered if the incoming/
outgoing is not accepted but call is terminated.
`terminate` is triggered if the accepted is terminated.
`incoming_held` is triggered if the incoming call is held.

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

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

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