Stop exporting systemserverclasspath_fragment when targeting S

Previously, when targeting the S release the generated sdk snapshot
would contain prebuilt_systemserverclasspath_fragment modules even
though they were only added in T.

This allows SdkMemberTypes to specify the set of target build releases
they support and ignores them when targeting an unsupported target
build release.

Test: m nothing
      packages/modules/common/build/mainline_modules_sdks.sh
      # Check that the for-S-build snapshots do not include SSCPFs.
Bug: 237718221
Change-Id: I2df08c2fcebf9b866695d691572a9d3783758b17