code: @GuardedBy("lock")
    private void updateStats() {
      mInternalStatFs = updateStatsHelper(mInternalStatFs, mInternalPath);
      mExternalStatFs = updateStatsHelper(mExternalStatFs, mExternalPath);
      mLastRestatTime = SystemClock.uptimeMillis();
    }
docs: (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).