Add no-op systemserverclasspath_fragment module.

This would allow to start introducing these modules for apexes that
contribute to SYSTEMSERVERCLASSPATH.

In follow up, it will be evolved:
- platform_systemserverclasspath would have "fragments" property to
  list all individual systemserverclasspath_fragments;
- systemserverclasspath_fragment would have "contents" property to list
  contibuting java libs;
- systemserverclasspath_fragment would generate non-empty
  classpaths.proto config within individual apexes.

Bug: 180105615
Test: m nothing
Change-Id: Ibaaa3fae5f1eab9a41ceecc1214a53be6bbc8ba6