ui: Include one or more perf tracks in area selection.

- factor out some of the code in aggregation_controller, so that
it can be reused in flamegraph_controller;
- if area selection changes, call the flamegraph_controller;
- modify FlamegraphState, so it can contain start timestamp, end
timestamp and array of upids;
- in FlamegraphDetails, store a boolean indicating if the area has
changed.

Examples:
a)select part of a perf sample:
https://screenshot.googleplex.com/8TGQYTadUscnTUt
b)select perf samples across multiple processes:
https://screenshot.googleplex.com/3HgpSSQPGRMzjVw

Bug: b/195934783
Change-Id: I2d17b10eb997b37f6d0268f04530314b1aacb11a