[Audiosharing] Returns BluetoothDevice when fetchConnectedDevicesByGroupId

CachedBluetoothDevice can change its member variable mDevice, so we can
not rely on the CachedBluetoothDevice#getDevice when we add source to
it. This change will return BluetoothDevice instead of
CachedBluetoothDevice when fetchConnectedDevicesByGroupId, so that we
won't add source to unintended BluetoothDevice via
CachedBluetoothDevice#getDevice.

Fix: 350877510
Test: atest
Flag: com.android.settingslib.flags.enable_le_audio_sharing
Change-Id: I4ee41b8f1449e7176f0a05a7dd4e59034c161824