Feature request: Show CPU frequency / on-off state

CPU governors make it really hard to see problems with performance on modern mobile devices — the data lies to you ([article](https://medium.com/flutter/performance-testing-of-flutter-apps-df7669bb7df7)). One solution is to scale lock the device, but many people won't have the patience or resources to do it.

The next best thing is to _see_ when CPU shifts into higher gear (or when a previously-disabled CPU is switched on). It would be awesome to show this in the timeline.

// cc @liyuqian

The issue names the timeline as the desired entry point but identifies no files, tests, or implementation path. First trace how timeline data is collected and rendered, then determine the platform-specific sources for CPU frequency and on/off state; done means those changes are visible in the timeline across supported mobile devices.

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