Add boot_images to apex

Previously, the apex module had to hard code behavior specific to the
art apex module in order to include the art boot image. This change
adds support to the apex module to allow the boot images to be
specified per apex.

In combination with a change to add the "art-boot-image" to the ART
apex this allows the custom code for handling the art boot image in
apex to be removed.

That custom apex code also included the logic to ensure that the
GlobalSoongConfig was initialized for use by the dex_bootjars
singleton. That logic has been moved from the APEX to the boot_image
module. That ensures that it will be run if and only if a boot_image
module is present in the checked out repos. So, limited manifest
checkouts which do not contain the art or frameworks/base repos (which
is where the boot_image modules are defined) will not attempt to run
this logic, which would fail because dex2oat would not be present.

Bug: 177892522
Test: m droid
Change-Id: I02d25fbef6e864e31eb5e0f4eb50358c79486db0