Move BatteryStats and StatsCompanionService to use NetworkStatsService.

This CL is a manual merge of http://ag/c/6015966/3.

Bug: 113122541
Bug: 120145746
Test: atest FrameworksNetTests
Test: manual test: verified that BatteryStats are correctly accounting
for VPN traffic.
Change-Id: I5b07ce70ac58bdcbebc3114bfe9fd411469d57af
Merged-In: I230c1edbf64cfeb3dbb560db368b5e420f7b79a4