Properly check if BT is off when shutting down radios

Use BluetoothAdapter#getState() instead of
BluetoothAdapter#isEnabled() when checking if Bluetooth has turned
off, as isEnabled() is set to false early in the shutdown process
and getState() provides a better indication of the state of BT.

Bug: 27354612
Change-Id: Ic7828f0726491d49c9a14ba5d654b24f66743662