在Android中，可以使用SharedPreferences或者SQLite数据库来实现Firebase用户在更新之间的持久性存储。以下是两种方法的示例代码：

1. 使用SharedPreferences：

// 保存用户数据
SharedPreferences sharedPreferences = getSharedPreferences("user_data", Context.MODE_PRIVATE);
SharedPreferences.Editor editor = sharedPreferences.edit();
editor.putString("username", "John");
editor.putString("email", "john@example.com");
editor.apply();

// 读取用户数据
SharedPreferences sharedPreferences = getSharedPreferences("user_data", Context.MODE_PRIVATE);
String username = sharedPreferences.getString("username", "");
String email = sharedPreferences.getString("email", "");

2. 使用SQLite数据库：

首先，创建一个用户类User，具有相应的属性和方法：

public class User {
    private int id;
    private String username;
    private String email;

    public User() {}

    public User(String username, String email) {
        this.username = username;
        this.email = email;
    }

    // Getter and Setter methods
    // ...
}

接下来，创建一个SQLiteOpenHelper类，用于创建和管理数据库：

public class UserDatabaseHelper extends SQLiteOpenHelper {
    private static final String DATABASE_NAME = "user_db";
    private static final int DATABASE_VERSION = 1;
    private static final String TABLE_NAME = "user_table";
    private static final String COLUMN_ID = "id";
    private static final String COLUMN_USERNAME = "username";
    private static final String COLUMN_EMAIL = "email";

    public UserDatabaseHelper(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        String CREATE_TABLE = "CREATE TABLE " + TABLE_NAME + "("
                + COLUMN_ID + " INTEGER PRIMARY KEY AUTOINCREMENT,"
                + COLUMN_USERNAME + " TEXT,"
                + COLUMN_EMAIL + " TEXT" + ")";
        db.execSQL(CREATE_TABLE);
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        db.execSQL("DROP TABLE IF EXISTS " + TABLE_NAME);
        onCreate(db);
    }

    public void addUser(User user) {
        SQLiteDatabase db = this.getWritableDatabase();
        ContentValues values = new ContentValues();
        values.put(COLUMN_USERNAME, user.getUsername());
        values.put(COLUMN_EMAIL, user.getEmail());
        db.insert(TABLE_NAME, null, values);
        db.close();
    }

    public User getUser() {
        SQLiteDatabase db = this.getReadableDatabase();
        Cursor cursor = db.query(TABLE_NAME, null, null, null, null, null, null);
        User user = null;
        if (cursor.moveToFirst()) {
            int idIndex = cursor.getColumnIndex(COLUMN_ID);
            int usernameIndex = cursor.getColumnIndex(COLUMN_USERNAME);
            int emailIndex = cursor.getColumnIndex(COLUMN_EMAIL);
            int id = cursor.getInt(idIndex);
            String username = cursor.getString(usernameIndex);
            String email = cursor.getString(emailIndex);
            user = new User(username, email);
            user.setId(id);
        }
        cursor.close();
        db.close();
        return user;
    }
}

最后，可以在应用程序的适当位置使用上述代码来保存和读取用户数据：

// 保存用户数据
User user = new User("John", "john@example.com");
UserDatabaseHelper databaseHelper = new UserDatabaseHelper(this);
databaseHelper.addUser(user);

// 读取用户数据
UserDatabaseHelper databaseHelper = new UserDatabaseHelper(this);
User user = databaseHelper.getUser();
String username = user.getUsername();
String email = user.getEmail();

以上代码示例了使用SharedPreferences和SQLite数据库来实现Firebase用户在更新之间的持久性存储。你可以根据自己的需求选择其中一种方法来实现。