floss: Shutdown DBus connection to prevent sysprops crash

It is possible to receive a DBus message when the adapter is disabled.
At this point, the stack manager and related threads have been stopped,
and the sysprop module has been shut down. Therefore, calling sysprop
via DBus, e.g. by BluetoothGatt.StartScan, might crash.

This CL prevents the crash by shutting down DBus connection before
disabling the adapter.

Bug: 308506915
Test: m -j
Test: Spam the following dbus message while powering off/on bluetooth
      via UI. Verify there is no crash.
      dbus-send --system --print-reply --dest=org.chromium.bluetooth \
      /org/chromium/bluetooth/hci0/gatt \
      org.chromium.bluetooth.BluetoothGatt.StartScan \
      byte:1 array:byte: array:byte:

Flag: EXEMPT floss only change

Change-Id: Ib6905bc24ec2584bb85c924b754531f8877b800b