Fixed app crash while calculating storage space
Bug appears to be with devices that have non-primary internal storage such as the
MyTouch 4g.  For devices such as these, the StorageMeasurement class skips measuring
media such as Pictures and Music on them.  However StorageVolumePreferenceCategory
still attempts to read the media measurement data resulting in an NPE.  Resolved the
issue by skipping this step in StorageVolumePreferenceCategory for non-emulated internal
storage and removed the preference categories for them.