[Session] Rely on agent session change event for persisting resumed session

[gaearon]

Extracted from #3728.

See two individual commits.

• 7095822: When we resume session, we don't want to block the UI so we assume it'll come through. Still, we kick off a resumeSession request in background. When that request comes back, we want to persist the new session if it's different from what we've stored. Turns out, we don't actually need to write this logic explicitly because the onAgentSessionChange handler already has the logic to handle session updates — and the agent will call it. So just kicking off a resume is sufficient on its own. Errors would be reported to onAgentSessionChange as well.
• 529d1fe: Previously, we had an optimization that avoids a state update right after startup if the resumed session is the same as what we've stored. We've lost this optimization in the last commit so let's re-add it here.

Test Plan

1. Toggle 2FA setting in a different browser window.
2. Reload and observe that the change is being picked up on resume via onAgentSessionChange.
3. Reload and observe that next resumes are able to bail out of the setState due to identical payload.

[render]

Your Render PR Server URL is https://social-app-pr-3836.onrender.com.

Follow its progress at https://dashboard.render.com/web/srv-coq4f5a1hbls73fl867g.

[github-actions]

Old size	New size	Diff
6.86 MB	6.86 MB	-304 B (-0.00%)

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[estrattonbailey]

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