Show USB flash drive in storage spinner when it's unmounted

Before this change, USB flash drive is removed from storage spinner
when it's unmounted (by option menu 'Eject'), however, USB flash
drive will show in storage spinner when Storage Settings page is
resumed again.

After this change, USB flash drive will show in storage spinner
even if it's unmounted. It will be removed until it's unplugged
from device.

Bug: 225095144
Test: manual visual
      Insert / Eject / Mount / unplug an USB flash drive and
      observe UI behavior.
Change-Id: I6b81766216c7850b018997f2de12a19d9d8675be