Add loader for net cycle data for specific Uid.

- Add the loader to retrive usage data for a particular uid.
- restructure the NetworkCycleData and the loader:
  - NetworkCycleData now becomes the base class of the cycle data. It
  only contains the cycle start/end and the total usage.
  - Add NetworkCycleChartData which has the bucketized usage for
  plotting the usage chart.
  - Add NetworkCycleDataForUid which shows detail usage for a specific
  app.

Bug: 111751694
Test: make RunSettingsLibRoboTests
Change-Id: Ifd95445730429373dc45aaceae37eb0d79b4377c