fix #2048 autosave edittext

fix #2048 autosave edittext
demo: https://cloudup.com/cwprpfKkMFg
+1 on using a separate class for this table, but I'm wondering whether this table should be in a separate database as well (perhaps one shared with the [suggestions table](https://github.com/wordpress-mobile/WordPress-Android/pull/1984)?). That way you can update the db version if the schema changes without affecting post-related data.