Generate prebuilt_systemserverclasspath_fragment.

Bug: 194150908
Test: m nothing
Test: manual - 1. Patch aosp/1835087.
  2. m out/soong/.intermediates/art/build/sdk/art-module-sdk/common_os/snapshot/Android.bp
  3. See prebuilt_systemserverclasspath_fragment modules and
     systemserverclasspath_fragments in an sdk_snapshot module.
Change-Id: I7876b077b0b4ed190f01dbfa9533de04ea32d524