Save the user's reading position in the home timeline (#3614)

- Add a field to AccountEntity to hold the reading position
- Provide a method to save in the viewmodel to save the position
- Save the position when TimelineFragment pauses

Does not restore the position yet.

app/src/main/java/com/keylesspalace/tusky/db/AppDatabase.java

@@ -16,6 +16,7 @@
 package com.keylesspalace.tusky.db;
 
 import androidx.annotation.NonNull;
+import androidx.room.AutoMigration;
 import androidx.room.Database;
 import androidx.room.RoomDatabase;
 import androidx.room.migration.Migration;
@@ -29,9 +30,20 @@ import java.io.File;
 /**
  * DB version & declare DAO
  */
-@Database(entities = { DraftEntity.class, AccountEntity.class, InstanceEntity.class, TimelineStatusEntity.class,
-                TimelineAccountEntity.class,  ConversationEntity.class
-        }, version = 48)
+@Database(
+    entities = {
+        DraftEntity.class,
+        AccountEntity.class,
+        InstanceEntity.class,
+        TimelineStatusEntity.class,
+        TimelineAccountEntity.class,
+        ConversationEntity.class
+    },
+    version = 49,
+    autoMigrations = {
+        @AutoMigration(from = 48, to = 49)
+    }
+)
 public abstract class AppDatabase extends RoomDatabase {
 
     public abstract AccountDao accountDao();