Mark TestNetworkManager as module API

The API surface is planned to move to the Connectivity module, meaning
that it will be maintained across multiple Android releases.
This is incompatible with TestApi semantics, which are designed to be
removable across releases. Effectively the APIs become System/module
API, so mark them as such considering that TestApi is not supported by
modules.

Test: m
Bug: 174436414
Change-Id: Icd32fcbb65e9a4bb2b67cb9da7c971281be0781a