Update App battery usage page and add tests.

- Add tests
- Remove duplicated "Set battery usage for apps" text
- Clean up some code

Screenshot: https://screenshot.googleplex.com/8LHhyxFA6X4ZFkT.png

Test: make RunSettingsRoboTests -j40
Bug: 244995036
Change-Id: Iaac9b0af10f541100e91b10affacf3307bd5edca