Do not throw exception when MeasurementDetails.mediaSize is empty

It is legitimate for StorageMeasurement.measureExactStorage to return without any user keys
added to the returned MeasurementDetails, when the following is true:

mVolume == null || !mVolume.isMountedReadable()

Bug: 22394495
Change-Id: Ie40bd468a487208d98dc56aae41fdad7098f41a5

src/com/android/settings/deviceinfo/PrivateVolumeSettings.java

@@ -533,8 +533,7 @@ public class PrivateVolumeSettings extends SettingsPreferenceFragment {
                 }
             }
         } else {
-            throw new IllegalStateException(
-                    "MeasurementDetails mediaSize array does not have key for user " + userId);
+            Log.w(TAG, "MeasurementDetails mediaSize array does not have key for user " + userId);
         }
         return total;
     }