Mark connectivity test utilities 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 SystemApi, so
mark them as such considering that TestApi is not supported by modules.

Remove the NetworkCapabilities.getCapabilities test API that is not
essential for tests, and mark other APIs as module API.

Test: m
Bug: 174436414
Change-Id: Ic04551a9874b3d756cf5e6c77ceabfc7c85d52a6