Export boot image files from prebuilt_apex/apex_set

Previously, the prebuilt art-bootclasspath-fragment did not provide any
boot image files. That meant it was impossible to build any module that
requires access to those files from prebuilts, e.g. any module that
needs to be dexpreopt-ed.

This change enables that module to retrieve those files from the
prebuilt_apex.

Bug: 177892522
Bug: 189298093
Test: m nothing
      m droid SOONG_CONFIG_art_module_source_build=false SKIP_BOOT_JARS_CHECK=true
      - the previous command does not work but this change does fix one
        of the issues reported.
Change-Id: I1d4d9545172d79282918130df6b9aa55471bffc1