应用场景：仅限调用StatFs.java获取才能实现此功能
frameworks\base\core\java\android\os\StatFs.java

import android.os.Environment;

String mPath = "";
  String ePath = "";

public StatFs(String path) {
        mStat = doStat(path);
++      mPath = path;
++      ePath = Environment.getExternalStorageDirectory().getPath();
    }

public long getBlockCountLong() {
++      if(mPath.equals(ePath)){
++          return 4194304;
++      }else{
            return mStat.f_blocks;
++      }
    }