🎨 Palette: Improve empty state UI in SessionListActivity

[yuga-hashimoto]

💡 What: Replaced the plain text empty state in SessionListActivity with a more visually appealing Column containing a chat icon, centered text, and better spacing.
🎯 Why: To make the screen look more polished and provide a friendlier user experience when a user has no active sessions, instead of a stark, unstyled text block.
📸 Before/After: Before: Plain text "No conversations yet." After: A semi-transparent primary-colored chat icon with slightly larger, centered text below it.
♿ Accessibility: The added decorative icon explicitly sets contentDescription = null to prevent redundant screen reader announcements, since the accompanying text already conveys the "no sessions" message.

---

PR created automatically by Jules for task 11573610812375713412 started by @yuga-hashimoto

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.