Auto-include device_manifest.xml when necessary

It doesn't make sense to specify a DEVICE_MANIFEST_FILE but the
build not including it. The conditional definition of the module
makes it problematic to have it listed in the PRODUCT_PACKAGES
of a generic mk.

Test: build mainline_arm64 + mainline_system_arm64
Change-Id: I40e435ce114263ceabbd373d3d719318c592fc13