Add support for tracking PANS data usage

Adds support for tracking oemManagedNetwork types (OEM_PAID and
OEM_PRIVATE).

Bug: 174485293
Test: atest NetworkTemplateTest NetworkStatsCollectionTest
NetworkStatsObserversTest NetworkStatsServiceTest NetworkIdentityTest
Test: verify that the bytes go up in PansTest app only when
`ping -I eth2 8.8.8.8` is run as root.

Change-Id: I78660a835f16265ba8e44bb84332c6633dd72b84