Add logic to ensure the ART APEXes are built from source in coverage

builds.

This CL is a nop since the default is still true, but it gives us a way
to toggle it without affecting coverage builds.

Test: env NATIVE_COVERAGE=true build/soong/soong_ui.bash \
        --dumpvar-mode SOONG_CONFIG_art_module_source_build
      env CLANG_COVERAGE=true build/soong/soong_ui.bash \
        --dumpvar-mode SOONG_CONFIG_art_module_source_build
      env NATIVE_COVERAGE=true CLANG_COVERAGE=true \
        build/soong/soong_ui.bash \
        --dumpvar-mode SOONG_CONFIG_art_module_source_build
  check that all these print "true" when the default in the "else"
  clause is "false".
Bug: 172480615
Change-Id: I648ed911773c59a5b2a117dbf3ddcb2ef338830d