Define prebuilt xml files as prebuilt_etc modules

These files were installed with PRODUCT_COPY_FILES. Instead of using
the PRODUCT_COPY_FILES define them as prebuilt_etc modules and
install them.

Different from the other modules in this directory, these new modules
are for system files.

Bug: 314933937
Test: m aosp_cf_system_x86_64
Change-Id: I3a8fb0046bbadcd5fcd5c2ded4d3b745b71e0f47