Keep persistent ActivityStates for everything on the stack.

Keep persistent ActivityStates for everything on the stack.

This fixes the cases where temporarily going to another full-screen
activity failed to restore the in-progress state of the previous one.
In particular, nipping out to the utilities menu from composing a
post, and reading the server rules during account registration.

It's a shame; I'd have preferred to be deleting and recreating state
objects all the time, to ensure they had a well-defined amount of
internal state. But this needed less organisation. Perhaps I can come
back and do it better another time.