Add an apex_contributions_defaults module to module sdk

This metadata module will contain a flattened list of all the soong
modules contributed by this module to the rest of the build. This is
supported in V and above.

Bug: 326246905
Test: m art-module-host-exports
Test: unzip -p out/soong/mainline-sdks/art-module-host-exports-current.zip Android.bp | grep -A 15 apex_contributions_defaults
apex_contributions_defaults {
    name: "art-module-host-exports.contributions",
    contents: [
        "prebuilt_art.module.api.annotations",
        "prebuilt_dex2oat",
        "prebuilt_dex2oatd",
        "prebuilt_dexdump",
        "prebuilt_hiddenapi",
        "prebuilt_oatdump",
        "prebuilt_profman",
        "prebuilt_veridex",
        "prebuilt_libartpalette",
        "prebuilt_libartbase",
    ],
}

Change-Id: Iba43a9da5430adfc0a5fd9b5dc27b5d89c5eacb8