Fix setActiveDevice(null)

BluetoothAdapter does not allow to remove a device with a
`setActiveDevice(null)`, instead we should call `removeActiveDevice()`

```
*** FATAL EXCEPTION IN SYSTEM PROCESS: BluetoothRouteManager
java.lang.IllegalArgumentException: device cannot be null
  at android.bluetooth.BluetoothAdapter.setActiveDevice(BluetoothAdapter.java:1990)
  at com.android.server.telecom.bluetooth.BluetoothDeviceManager.disconnectAudio(BluetoothDeviceManager.java:250)
  at com.android.server.telecom.bluetooth.BluetoothRouteManager$AudioConnectedState.processMessage(BluetoothRouteManager.java:388
```

Bug: 204727408
Fix: 204727408
Test: N/A
Tag: #stability
Change-Id: I0f12297370a2f633f881518cdbdfcdb042d11b4f