In addition to existing 100, 25, and 0 battery charge percentage icons, SF Symbol 3 introduced 75 and 50 for devices running iOS 15.
- [ ] Use the additional symbols when available on iOS 15
- [ ] Try to eliminate the "unknown state" in the current codebase (okay to use `precondition` to assert certain conditions or met in the catch-all branch, or `fatalError` for the impossible branch)