Transitively add APEX variants for contents of prebuilt_apex/apex_set

This is part of the work needed to allow the exported_java_libs
property to be replaced by exported_bootclasspath_fragments.

Bug: 187266082
Test: m nothing
      m SOONG_CONFIG_art_module_source_build=false nothing
Change-Id: I2c1d70a390200b93163f9799719290c9d55a041c