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
289ba40237