Allow overriding SOONG_CONFIG_art_module_source_build to false.

Useful for local testing etc.

Cherry-picked from http://ag/15754315.

Test: env SOONG_CONFIG_art_module_source_build=false \
          MODULE_BUILD_FROM_SOURCE=true \
        build/soong/soong_ui.bash \
          --dumpvar-mode SOONG_CONFIG_art_module_source_build
  return false
Bug: 172480615
Change-Id: I50187408a01e0c6f5ecea12fe3a423e1c5d93612
Merged-In: I50187408a01e0c6f5ecea12fe3a423e1c5d93612