Use NetworkStatsService to get stats for apps

Move the native implementation of TrafficStats to NetworkStatsService
and apps need to get the NetworkStatsService binder interface from
system_server in order to get the network usage stats since boot. This
implementation can hide the detail of retrieving network traffic
information from apps and the NetworkStatsService can choose which
methoed it use to get the data depending on the kernel version of the
device.

Bug: 30950746
Test: CtsNetTestCases -t android.net.cts.TrafficStatsTest
Change-Id: I53bbefd19aa0b783b9b4b42ea4d76db3e9ec07a3