floss: Fix the battery level update in the BAS

To receive the battery level char update notification
from the peer device, we need to configure CCCD before
calling the register_for_notificatino in the GATT client.

Bug: 394986741
Tag: #floss
Test: m Bluetooth
Flag: EXEMPT floss only changes
Change-Id: Iac6ea73aa9fb170e176d0f559ca4a8979a63ea46