Improve UX and Timeline API for Flutter performance viewer task timelines 2

This is a continuation of a previous issue I've seen [here](https://github.com/flutter/devtools/issues/6314).

I am trying to use the Performance tool (I use VS code flutter and dart extensions 3.92). I am quite experienced with recording traces using the Perfetto C++ API.

My Issues are as follows:

### UX:
1. It is very hard to understand what period of time we are recording. For instance, what does pause do? what does export data exports?
usually the user would like to record a certain period of time. a **simple start + stop** recording would be helpful. maybe even with a simple clock that displays how much time we are recording.
2. I don't understand what **Clear all** does. after pressing it, I get empty traces if I start recording again.
3. likewise, pause is very unintuitive. what does it pause exactly.
4. It is very hard to filter out irrelevant data from being recorded. What if I only wish to see my traces?
5. In general, it is as if this tool is meant to be used by flutter engine / framework developers and not simply flutter developers who wish to profile their own code.

### Timeline API:
1. it is very hard to predict on which perfetto "track" my traces will end up on. for instance: calling Timeline.startSync / Timeline.finishSync will (maybe) end up on the flutter ui track, whereas Timeline.instantSync will end up on a different Track.
2. TimelineTask creates a Track for each layer. this is undesired.
3. the documentation is very lacking. what does the Map argument in startSync is meant for? what can we do with it?

The most fundamental thing to add to the API is the the ability to have my traces on a separate track. `perfetto::Track(_trackId)`.
This way I can view all of them on a single track in the perfetto trace. and not in between all the flutter traces I am not interested in.

Start with the previous issue #6314 and the Performance tool's recording controls, then inspect Timeline.startSync, Timeline.finishSync, Timeline.instantSync, and TimelineTask behavior. Define a focused change before implementation; done should make recording scope and controls clear, group user traces on a separate Perfetto track, and document the Map argument.

索引モデルが issue の本文から書いたものです。