Make apex variants of prebuilt apex exported modules available in make

Previously, the apex variants of modules exported from prebuilt_apex
and apex_set modules were not exported to make. Neither by the modules
themselves or by the prebuilt apex module. The master-art build relied
on the platform variants of the conscrypt and core-icu4j libraries
being exported to make so that they could be used by vogar.

Unfortunately, a change to export the prebuilt_bootclasspath_fragment
modules that contain the conscrypt and core-icu4j prebuilt libraries
from the corresponding prebuilt_apex prevented the platform variants
of those libraries from being exported at all which broke the
master-art builds as it only has prebuilts of those modules. It did not
break the aosp/master build as that has source modules.

The difference between the two builds is that the apex module type
makes its contents available in make but the prebuilt_apex/apex_set
module types do not.

This change causes the prebuilt_apex/apex_set module types to behave
more like the apex module type by making its exported libraries
available in make.

Test: m droid
      - in aosp/master
      art/tools/buildbot-build.sh --target --installclean
      - in master-art and aosp/master
Change-Id: I57537d17d4920d37d896ed491c6aaa15302cb25d