[Alerts page] move alert unified search bar to the top

[maryam-saeidi]

Implement after https://github.com/elastic/kibana/issues/177431 to avoid conflict.

🍒 Summary

Based on the new design, we want to move the search bar to the top and show the active alerts by default.

🎯 Acceptance Criteria

• Move the unified search bar to the top of the page
• Set active as the default status
• Refactor status filter based on design

[elasticmachine]

Pinging @elastic/obs-ux-management-team (Team:obs-ux-management)