Use CopyOnWriteArrayList to store list of NetworkStatsProviderCb

In NetworkStatsService, mStatsLock will be held when
iterating through the provider list. This is to protect the list
from concurrent broadcast that triggered by
NetworkPolicyManagerService. This is not good since the binder
call is oneway, it does not make sense to block every
access to the providers.

This change also remove unuse variable and reduce verbose log.

Test: atest FrameworksNetTests TetheringTests
Bug: 150418178
Change-Id: If74e9f2ea597a0d5ae4668c3358bc687f342bbb5