[SecuritySolution] Update the style for timeline's unsaved badge

[angorayc]

https://docs.google.com/document/d/18moX6psls83LX-HfvLN244LDZZzlf8pkt5bxfqT4nM8/edit?tab=t.0#heading=h.qkwmjv8mj937

The warning badge is too prominent on collapsed timeline

The ‘unsaved’ badge on the timeline is too prominent when the timeline is collapsed. The warning colour is deliberately prominent to draw the user’s eye, but in the context where the timeline is not open, from a UX perspective, it should be downgraded in its importance.
Suggested fix
Make the unsaved badge hollow when the timeline is collapsed and the user is focused on other things, and only show the filled version when it is an important warning, like when the timeline is open.

[elasticmachine]

Pinging @elastic/security-threat-hunting-investigations (Team:Threat Hunting:Investigations)