last_active_ago is updated on sync when it shouldn't be

[dbkr]

Description

The last_active_ts is updated for a user when they perform a regular sync request where their previous presence is 'online'. It looks like synapse intends to only update this when a sync is performed with set_presence = online and the previous presence state is online, and the spec says it should only be updated on a proactive event from the user.

This is because set_state is called in order to set the new presence state and this method always updated last_active_ago.

Found while working on #12213 but I think this will need another inter-process API added to fix.

Steps to reproduce

• Have two users, leave one syncing.
• On the other user, perform a sync request
• Notice on the first sync stream that the last_active_ago value for the other user reflects the the sync you just did.

Version information

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