Bluetooth: Host: Fix GATT delayed store

When `CONFIG_BT_SETTINGS_DELAYED_STORE` is enabled (by default it is),
the local GATT server will delay the write to flash of the CCC and CF
values. The delay is defined by `CONFIG_BT_SETTINGS_DELAYED_STORE_MS`.
If a disconnection happen before that delay, the `bt_gatt_disconnected`
will reschedule the delayed store operation. But that operation will not
happen before `bt_gatt_disconnected` is complete, at this moment, the
CCC and CF values will have already been cleared.

To fix this issue, the delayed store operation is now done during the
`bt_gatt_disconnected` function.

Signed-off-by: Théo Battrel <theo.battrel@nordicsemi.no>