Allow more than one source of tethering statistics.

Currently, netd is the only source of tethering statistics.
In order to support multiple sources, define a new
ITetheringStatsProvider interface that can be registered with
NetworkManagmentService. Convert the existing code into the
first ITetheringStatsProvider.

Bug: 29337859
Bug: 32163131
Test: builds, boots
Test: tethering stats continue to be collected
Change-Id: Ie1b5a5e47ae4bf5af922365b09fa241e834236e4