@GuardedBy("lock")
  private void updateStats() {
    mInternalStatFs = updateStatsHelper(mInternalStatFs, mInternalPath);
    mExternalStatFs = updateStatsHelper(mExternalStatFs, mExternalPath);
    mLastRestatTime = SystemClock.uptimeMillis();
  }
(Re)calculate the stats. It is the callers responsibility to ensure thread-safety. Assumes that
it is called after initialization (or at the end of it).
fbcore/src/main/java/com/facebook/common/statfs/StatFsHelper.java