Use prebuilts of the ART Module by default.

This means prebuilts located in
/vendor/unbundled_google/modules/ArtGooglePrebuilt and
/prebuilts/module_sdk/art are used instead of sources in /art,
/libcore, and /libnativehelper.

Sanitizer and coverage builds builds are excluded, as are builds using
the products art_module_*, armv8, arm_krait, and others in
device/generic/art.

To keep using ART and libcore sources in local builds, set
SOONG_CONFIG_art_module_source_build=true. See art/build/README.md for
further info.

Merged-In set to http://ag/15107826 to avoid merging downstream.

Test: presubmits
Test: vendor/google/build/build_mainline_modules.sh
  with a line
    build/soong/soong_ui.bash --dumpvar-mode \
      SOONG_CONFIG_art_module_source_build
  directly before the module build commands in
  build_unbundled_mainline_module.sh and check that it's still true.
Bug: 192542393
Merged-In: I8990b726affeb08eabf130d20be45129a7c1c58d
Change-Id: Id719640d17effca107af82de69e45514bf3bd234