Preparing to support multi user. Use the database to store accounts' info. LoginActivity is successfully refactored. Any other features are unavailable for now.

app/src/main/java/Account/AccountDao.java

@@ -0,0 +1,22 @@
+package Account;
+
+import androidx.lifecycle.LiveData;
+import androidx.room.Dao;
+import androidx.room.Insert;
+import androidx.room.OnConflictStrategy;
+import androidx.room.Query;
+
+@Dao
+public interface AccountDao {
+    @Insert(onConflict = OnConflictStrategy.REPLACE)
+    void insert(Account account);
+
+    @Query("DELETE FROM accounts")
+    void deleteAllAccounts();
+
+    @Query("SELECT * FROM accounts WHERE username = :userName COLLATE NOCASE LIMIT 1")
+    LiveData<Account> getAccountLiveData(String userName);
+
+    @Query("SELECT * FROM accounts WHERE username = :userName COLLATE NOCASE LIMIT 1")
+    Account getAccountData(String userName);
+}