Workaround for existing packages check failure when the `generic`

product is used with ART prebuilts.

The prebuilt packages in packages/modules/ArtPrebuilt don't get
Android.mk entries in the `generic` product (as opposed to e.g.
`aosp_arm`), so the enforce-product-packages-exist check fails there.

Exclude the ART APEX modules from the check as a workaround. It's also
necessary to remove the check for superfluous allow list entries, since
which modules are actually missing depends on many other variables (see
runtime_libart.mk).

Test: m nothing TARGET_PRODUCT=generic \
      SOONG_CONFIG_art_module_source_build=false
Bug: 172480615
Bug: 182105280
Change-Id: Id67278615f3e7c64c7658bbf1b2aa71ed6b381da