By using MasterSwitchPreference, we make "battery saver" to have
two parts: left part is the clickable summary jumping to other
fragment while right part is a switch toggle.
Also remove the previous battery saver preference.
Bug: 34279051
Test: RunSettingsRoboTests
Change-Id: If36fa9741e413a9bbdd57fa67a2c15b6e457afd7