Fix only handling the last presence state for each user (#9425)

This is a small bug that I noticed while working on #8956.

We have a for-loop which attempts to strip all presence changes for each user except for the final one, as we don't really care about older presence:

https://github.com/matrix-org/synapse/blob/9e19c6aab4b5a99039f2ddc7d3120dd3b26c274b/synapse/handlers/presence.py#L368-L371

`new_states_dict` stores this stripped copy of latest presence state for each user, before it is... put into a new variable `new_state`, which is just overridden by the subsequent for loop.

I believe this was instead meant to override `new_states`. Without doing so, it effectively meant:

1. The for loop had no effect.
2. We were still processing old presence state for users.