[UI] improve UX for power rail visualization

I would like to propose a set of changes to improve UX for power rail visualization in the Perfetto UI. In #910 we improved UX for power rail aggregations. this issue aims to capture UX improvements to the presentation of the power rail tracks on the timeline.

current state of UI for reference (test/data/power_rails.pb):

I'm proposing a plugin to handle this with the following changes to behavior:
1. drop "power." prefix and "_uws" suffix from the track name.
2. give tracks proper units. This unit should change appropriately when switching from "value" or "delta" mode to "rate" mode (eg we should display "uW" as the unit for "rate" mode and not "uWS / S"
3. switch displayed units from uW to mW since in most common cases android phones will be operating in the 1-1000mW range.
4. Add track groups for rails that share a [subsystem](https://source.chromium.org/chromium/chromium/src/+/main:third_party/perfetto//protos/perfetto/trace/power/power_rails.proto;l=29) to provide better organization when there are large numbers of tracks.

is there any concerns with any of the above changes? i would be willing to contribute PRs to accomplish the above.