Bluetooth: MGMT: Add management security level changed event

Add an event on device security level or encryption type change to
let user space know which level is currently in use.
Reset security level to 0 on disconnection so further connections
will correctly report security level changes.

This will be used for BlueZ qualification automation.

Assisted-by: GPT:GPT-5.3-Codex
Assisted-by: Copilot:claude-sonnet-5
Signed-off-by: Frédéric Danis <frederic.danis@collabora.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>