Making "Battery Saver" use MasterSwitchPreference.

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