Fixed check for volume name.

There are scenarios - like when an SD Card is adopted as internal
storage - where the getBestVolumeDescription() returns null, in which
case we should use the StorageVolume description instead (or a default
value if such description is also null)>

Change-Id: Ia11c3f4069dfbe252c568210ee60b5d8c384d863
Fixes: 29198046