Bluetooth: Classic: HGP_AG: change `get_ongoing_call()` to async mode

Change the callback `get_ongoing_call()` of the AG from synchronous to
asynchronous mode. It will help to avoid the Bluetooth host stack be
blocked in the context of callback `get_ongoing_call()`.

Add a function `bt_hfp_ag_ongoing_calls()` to set the ongoing calls and
reply the AT command `AT+CIND?` after the callback `get_ongoing_call()`
has been notified.

Add a delayable worker to avoid the AT command `AT+CIND?` never being
replied. After the time exceeds
@kconfig{CONFIG_BT_HFP_AG_GET_ONGOING_CALL_TIMEOUT}, the response of
the AT command `AT+CIND?` will be replied.

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