Remove use of fake soong.variables.

Use the --soong-only flag instead to create it the proper way with the
config make step. This fixes build problems arising from the incomplete
soong.variables settings.

This partially relands https://r.android.com/1535202 so that we'll use
TARGET_PRODUCT=mainline_sdk in module SDK builds everywhere.
build-aml-prebuilts.sh isn't removed completely because we still need
the special case to create build_number.txt.

Test: art/build/build-art-module.sh --skip-apex && \
      packages/modules/ArtPrebuilt/update-art-module-prebuilts.py \
        --local-dist out/dist
  then check that the created prebuilt dex2oatd64 runs with the repro
  in b/190194345
Test: build/soong/scripts/build-mainline-modules.sh
  then
      prebuilts/runtime/mainline/update.py --local-dist .../out/dist
  in a master-art tree and check that art/build/build-art-module.sh
  works there
Bug: 190194345
Bug: 174315599
Change-Id: I86327b082b61c292f18c0a6b315d9add08002832