Add TEST_NETWORK_SERVICE to module API

TestNetworkManager is being unbundled; the TEST_NETWORK_SERVICE constant
is necessary for the module to register the manager with
registerContextAwareService.

Bug: 171540887
Test: m
Change-Id: I0690251ddfcdaa8a34830e110b75869a370de389
Merged-In: I0690251ddfcdaa8a34830e110b75869a370de389