Ensure primary boot image files are created before they are used

Previously, when building a framework boot image variant it added an
implicit dependency onto the first file in the primary boot image
variant to ensure that the primary boot image variant files that the
dex2oat command needs have been created. That works when generating
from source as in that case all the files for a boot image variant are
created by a single command. However, it does not work for prebuilts as
each prebuilt file will be copied into the required location by
separate copy commands.

This change adds all the files that the dex2oat command uses implicitly
when building an extension boot image as implicit dependencies.

Bug: 177892522
Test: m SOONG_CONFIG_art_module_source_build=false droid
      - the previous command only works in combination with a number of
        other build changes.
Change-Id: I183748fd17f8f3003890675b8c6bb9fcab331443