@VisibleForTesting
  void deleteOldStorageIfNecessary() {
    if (mCurrentState.delegate != null && mCurrentState.rootDirectory != null) {
      // LATER: Actually delegate this call to the storage. We shouldn't be
      // making an end-run around it
      FileTree.deleteRecursively(mCurrentState.rootDirectory);
    }
  }
Gets a concrete disk-storage instance. If nothing has changed since the last call, then the
last state is returned

@return an instance of the appropriate DiskStorage class
@throws IOException
imagepipeline-base/src/main/java/com/facebook/cache/disk/DynamicDefaultDiskStorage.java