Update and expose ModemActivityInfo

Update ModemActivityInfo to present a nicer API surface and expose it as
a SystemApi. Also change clients in BatteryStats to match the new
surface.

This is part 1. Part 2 will modify the methods in TelephonyManager to
use a more up-to-standards threading model.

Bug: 170427831
Test: atest ModemActivityInfoTest
Change-Id: I762ed949342861c75d94eafce88335e7bd8c9139