Bluetooth: Add handler of MGMT_OP_READ_ADV_MONITOR_FEATURES

This adds the request handler of MGMT_OP_READ_ADV_MONITOR_FEATURES
command. Since the controller-based monitoring is not yet in place, this
report only the supported features but not the enabled features.

The following test was performed.
- Issuing btmgmt advmon-features.

Signed-off-by: Miao-chen Chou <mcchou@chromium.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>