frameworks: improve semantics and documentation for asyncTraceForTrack*

This CL improves the documentation and semantic behaviour of asyncTraceForTrackBegin
to match how these methods were expected to be used. [1] misuses
the event as previously written because it provides overlapping
events with the same cookie: however, it also nests the events and due
to how Perfetto UI/trace_processor are implemented, this just ends up
*happening* to work out.

I will send some follow up CLs cleaning up usages in the broadcast code
where we should nest events instead of overlapping them.

Bug: 259535570
Change-Id: Idcf10bed7ed1228b0ba9aa62ef3f47169030cc6a