Lightweight checkin output for network stats.

Define and print a compact version of network statistics when dump
is requested with the "--checkin" flag.  Defaults to last 24 hours,
but included data can be tweaked with various flags.

Groups together detailed network identities into larger umbrella
terms like "mobile" and "wifi."

Bug: 18415963
Change-Id: I70cf9c828ea5c6e5bb6884837d3608f66fbad2e6