AudioDeviceInventory: reset SCO active device upon restore failure

BtHelper holds mBluetoothHeadsetDevice as the SCO active device, which
may fall out of sync with mConnectedDevices in AudioDeviceInventory
during audio server crash if it's not reset to null. In that situation
SCO audio ports can no longer transition to connected state even after
BT headset disconnect then reconnect.

Bug: 380772841
Flag: com.android.media.audio.as_device_connection_failure
Test: Connect BT headset, trigger audio HAL crash, reconnect BT headset
and verify SCO audio ports are back

Change-Id: Ie38f5e3e6d9707e4a2f04d78eb23b09641c71d9e