This is the first step to move BinderCallsStats implementation to the server package in order to have an implementation more specific to the system server (tracking charger on/off). Removes the volatile keywords in BinderCallsStats. Also exposes a clean API to StatsCompanionService instead of BinderCallsStats directly. Test: unit test Change-Id: Ied98c555acc8489420104098561b4608346509b5