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: I609e4bc646097766d86c8b444ec7322770f79688
diff --git a/Settings/src/com/android/tv/settings/device/StorageResetActivity.java b/Settings/src/com/android/tv/settings/device/StorageResetActivity.java
index 6b448d5..3e88b36 100644
--- a/Settings/src/com/android/tv/settings/device/StorageResetActivity.java
+++ b/Settings/src/com/android/tv/settings/device/StorageResetActivity.java
@@ -664,7 +664,7 @@
                 }
             }
         } else {
-            throw new IllegalStateException(
+            Log.w(TAG,
                     "MeasurementDetails mediaSize array does not have key for current user " +
                     ActivityManager.getCurrentUser());
         }