Change range of battery saver schedule seekbar

The minimum value which can be set as trigger for
Battery saver was changed from 10% -> 20% however the
settings UI was not aligned with this change.

ref: b0fe225a Update the battery saver scheduler minimum level

Update the UI to align with the minimum trigger
level.

Bug: 396444535
Test: manual, check minimum seekbar value after reboot.
Test: atest BatterySaverScheduleSeekBarControllerTest
Change-Id: I4baa766ff1cb78ce656ae340ce930094b9e124e0