Connection handle is 0xFFFF in attribute access callback when ble_gatts_notify() is called.

Whenever the server application calls `ble_gatts_notify` there is a characteristic read event (`access_cb`) and the connection handle passed to that event is 0xFFFF.

The issue exists because `ble_gatts_notify` calls `ble_gatts_notify_custom` with `txom` set to `NULL`.
In `ble_gattc_notify_custom`, if `txom` is `NULL`, it fills the buffer with data from the same attribute whose handle is passed to `ble_gatts_notify_custom`.
The way it does that is by calling the access callback of that attribute through `ble_att_svr_read_handle`, and it specifies the connection handle as `BLE_HS_CONN_HANDLE_NONE`, which is `0xFFFF`.

So server application gets an invalid connection handle when notifying a client.

Trace the ble_gatts_notify to ble_gatts_notify_custom path, then inspect how ble_gattc_notify_custom invokes ble_att_svr_read_handle and passes the connection handle to the access callback. Reproduce a notification from a server application and verify that the callback no longer receives BLE_HS_CONN_HANDLE_NONE (0xFFFF).

Written by the indexing model from the issue text.