Remplace NetworkIdentity.OEM_* with NetworkTemplate.OEM_MANAGED_*

Remove hidden APIs and replace the constants with SystemApi that are
exposed by NetworkTemplate.
Also fix typo and remove getElapsedRealtime() which is a hidden API
from StatsPullAtomService.

Bug: 210073043
Test: TH
Change-Id: I8b980417faf776b9b7b0fbcc4f04525b516452a0