Ensure CPU profiler does not clear CPU samples after isolate exit event

[kenzieschmoll]

I saw this bug in a demo. The user ran their app with pause isolates on exit, pulled a profile for an isolate spawned with compute, and then resumed the isolate. The profile disappeared from the profiler after clicking resume.

[srawlins]

ProfilerScreenController maintains a map of profile data per isolate (_previousProfileByIsolateId), preserving CPU profile data across isolate switching and lifecycle events.

If there is more work to do here, please let us know by filing a new issue with up to date information. Thanks!