The following code is similar to what you just wrote. Modify this code to only write the file if it does not already....

import java.io.InputStream;
import java.io.FileNotFoundException;
import java.io.IOException;

public class FileUtilities {

  public static boolean copyResult;

  public static void saveAssetImage(Context context, String assetName) {
    File fileToWrite = new File(context.getFilesDir(), assetName);
    AssetManager assetManager = context.getAssets();

    try {
      InputStream in = assetManager.open(assetName);
      FileOutputStream out = new FileOutputStream(fileToWrite);
      copyResult = copyFile(in, out);
    } catch(FileNotFoundException e) {
      e.printStackTrace();
    } catch (IOException e) {
      e.printStackTrace();
    }
  }

  private static boolean copyFile(InputStream in, FileOutputStream out) {
    // Copy magic intentionally omitted
    byte [] buffer = new byte [1024];
    int write;
    while (( write = in.read (buffer )) !=-1) {
      out.read (buffer, 0 ,write);
    }
    return true;
  }
}