Remove battery saver toggle from the Battery screen

- Also fix bug where the summary doesn't show up until the battery saver state
changes.

Bug: 74120126
Fix: 74400918
Test: manual
Test: b ROBOTEST_FILTER=BatterySaverControllerTest RunSettingsRoboTests
Change-Id: I03ceb8ee2a8d8c33da7ad220148ecc9553911661