Fix incorrect previous active device

Previous commit moves the code around and cause an issue that, in
headset service, the previous active device is passed to audio server
_after_ it got reset to null, causing the SCO port not removed.

Bug: 381169303
Bug: 315234036
Flag: com::android::bluetooth::flags::is_sco_managed_by_audio
Test: atest HeadsetServiceTest
Connect BT headset then disconnect, verify by command
adb shell dumpsys audio  | grep BluetoothActiveDeviceChanged

Change-Id: I437b90f009593bf3dc1770595df42683e1e130d6