@Override
  public boolean isExternal() {
    return mIsExternal;
  }
Instantiates a ShardedDiskStorage that will use the directory to save a map between keys and
files. The version is very important if clients change the format saved in those files.
ShardedDiskStorage will assure that files saved with different version will be never used and
eventually removed.

@param rootDirectory root directory to create all content under
@param version version of the format used in the files. If passed a different version files
    saved with the previous value will not be read and will be purged eventually.
@param cacheErrorLogger logger for various events
imagepipeline-base/src/main/java/com/facebook/cache/disk/DefaultDiskStorage.java