bluetooth: hfp_ag: Add Kconfig option for in-band ringtone

Add a new Kconfig option CONFIG_BT_HFP_AG_INBAND_RINGTONE to allow
enabling/disabling the in-band ringtone capability in HFP AG profile.

The in-band ringtone feature is now conditionally compiled based on
this configuration option. When disabled, the feature flags are set
to 0 and the bt_hfp_ag_inband_ringtone() API returns -ENOTSUP.

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