Collect background network data stats in BatteryStats

Keep the original values the same, to avoid messing
up existing pipelines and stats, and keep additional
counters for background traffic stats for mobile and
wifi.

Bug: 35677165
Test: cts-tradefed run cts-dev -m CtsDumpsysHostTestCases
Change-Id: I5f5ac3d5b659fe3d0b13703e6666a8751c5d9469