Adds an Observer interface to observes binder API calls.

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