Fix small bug where the mainline module list couldn't be overridden.

Test: env MAINLINE_MODULES=com.android.art \
        art/build/build-art-module.sh
Test: art/build/build-art-module.sh --skip-apex
Bug: 172480615
Change-Id: I040aeea5c7409993e5e8abd855e8cf342caee5fe
1 file changed