buildscripts: android.sh to clean the build when building HEAD^
buildscripts: android.sh to clean the build when building HEAD^

A gradle plugin [issue](google/protobuf-gradle-plugin#331) was seen for https://github.com/grpc/grpc-java/pull/6099. This PR try to workaround it.
grpc/grpc-java,dapengzhang0/grpc-java,stanley-cheung/grpc-java,ejona86/grpc-java,grpc/grpc-java,dapengzhang0/grpc-java,grpc/grpc-java,ejona86/grpc-java,stanley-cheung/grpc-java,stanley-cheung/grpc-java,carl-mastrangelo/grpc-java,carl-mastrangelo/grpc-java,dapengzhang0/grpc-java,stanley-cheung/grpc-java,carl-mastrangelo/grpc-java,dapengzhang0/grpc-java,carl-mastrangelo/grpc-java,ejona86/grpc-java,grpc/grpc-java,ejona86/grpc-java