bta: le_audio: Set connection state prior ProcessHciNotifAclDisconnected

This fixes the connection state when ProcessHciNotifAclDisconnected is called.
The ProcessHciNotifAclDisconnected calls other functions that check the
connection state and may see the device as still connected.

Bug: 357472821
Flag: Exempt; change already covered in unit tests
Test: atest bluetooth_le_audio_client_test
Change-Id: I5d0c503643eb2e91c30b796e50068e461b1ad9c9