[MS08] Move NetworkStats files to f/b/package/ConnectivityT

NetworkStatsService is going to be moved into ConnectivityService
module. Move all related files to packages/ConnectivityT so that
it can be easily migrate these files to connectivity module
after clearing the hidden API usages.

Bug: 197717846
Test: TH
Change-Id: Iead00832b5eb7b1dc40a92027c5a14ae8316b16c