Add exported_systemserverclasspath_fragments to prebuilt_apex rule.

This is for exporting the contents of systemserverclasspath_fragment for
dexpreopting.

Bug: 194150908
Test: manual - 1. Patch aosp/1818020 and aosp/1834534
  2. m SOONG_CONFIG_art_module_source_build=false com.android.art
Change-Id: I7d2d2e02869d8a523f7c0efbbff81706672a95c5