Add SQLite migration from version 2 to 3 (#667)

* Add migration for 2 -> 3

Author: jmshrv

android/src/main/java/vn/hunghd/flutterdownloader/TaskDbHelper.java

@@ -56,8 +56,12 @@ public void onCreate(SQLiteDatabase db) {
 
     @Override
     public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
-        db.execSQL(SQL_DELETE_ENTRIES);
-        onCreate(db);
+        if (oldVersion == 2 && newVersion == 3) {
+            db.execSQL("ALTER TABLE " + TaskEntry.TABLE_NAME + " ADD COLUMN " + TaskEntry.COLUMN_SAVE_IN_PUBLIC_STORAGE + " TINYINT DEFAULT 0");
+        } else {
+            db.execSQL(SQL_DELETE_ENTRIES);
+            onCreate(db);
+        }
     }
 
     @Override