Copy boot dex jars from prebuilt art-bootclasspath-fragment if preferred

Previously, the boot dex jars were only copied to the predefined
locations used by the build from the source art-bootclasspath-fragment
if it was preferred, otherwise no files were copied. That caused build
failures when attempting to build with ART prebuilts. This change
copies the files from prebuilts too.

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: I35b37355170546daf6ecac2134d1ca9a73d0e3bc