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

scripts/build-mainline-modules.sh

@@ -93,11 +93,17 @@ for product in "${PRODUCTS[@]}"; do
   done
 done
 
+# We use force building LLVM components flag (even though we actually don't
+# compile them) because we don't have bionic host prebuilts
+# for them.
+export FORCE_BUILD_LLVM_COMPONENTS=true
+
 # Create multi-archs SDKs in a different out directory. The multi-arch script
 # uses Soong in --skip-make mode which cannot use the same directory as normal
 # mode with make.
 export OUT_DIR=${OUT_DIR}/aml
-echo_and_run build/soong/scripts/build-aml-prebuilts.sh ${MODULES_SDK_AND_EXPORTS[@]}
+echo_and_run build/soong/scripts/build-aml-prebuilts.sh \
+  TARGET_PRODUCT=mainline_sdk ${MODULES_SDK_AND_EXPORTS[@]}
 
 rm -rf ${DIST_DIR}/mainline-sdks
 echo_and_run cp -R ${OUT_DIR}/soong/mainline-sdks ${DIST_DIR}