Breaking history writing out of BatteryStatsImpl

Bug: 242230266
Test: atest FrameworksServicesTests:BatteryStatsTests
NoNonSdkCheck: BatteryStats.getHistoryBaseTime was only used locally within BatteryStats itself.  It was assigned to an unused variable, so removing this method is a no-op
Change-Id: Ib132f3e0a13d639c02e51188358fb4525b00ca9c