Add time unit into battery usage chart and refine some UI

- add time unit into battery chart for 12/24 hours
- refine the radius size for chart bar
- use fixed font size for timestamp and percentage

screenshots:
24-hour format: https://screenshot.googleplex.com/3VNedE2nAx6LrXu
12-hour format: https://screenshot.googleplex.com/3tpTXjYfrasrenW
fixed-size: https://screenshot.googleplex.com/nPRKm7XCkmFzW6K

Bug: 194836607
Test: make SettingsRoboTests
Change-Id: I5b35b0fa808eb8a5f607962ee973c2896529b078