[Bug] User's state should be remembered between switching steps on Export Messages

[yanCode]

Expected:
as a user, I should be able to select some messages in Select Step , and then go to Preview Step. after I previewed the selected messages, I want to go back, so I can continue to select/deselect some messages based on previous selection.
Actual:
when switch back from Preview to Select, user's previous selection was overwritten by select all logic. not only that, if you the user has input any texts in the search box, the input also got lost.

I am creating a PR to this.