Add showChargingMethod in BatteryInfo

This toggle is used to decide whether to show charging method.
If it is off, then "Charging over ac" will become "Charging"

Bug: 36558216
Test: RunSettingsLibRoboTests
Change-Id: I98b1f7e7b77d7ce0a7e1aae29e55a33ac79ff0d7
(cherry picked from commit b3c33f63)