buildscripts: Increase memory for Gradle in Android CI
buildscripts: Increase memory for Gradle in Android CI

We've still been seeing random memory-related failures with the Android
CI, but it is nowhere near as severe as it was. But even when running
locally with "-Xmx512m -XX:MaxMetaspaceSize=512m" I get failures. Our CI
environment has lots of RAM; let's use it.

stanley-cheung/grpc-java,grpc/grpc-java,grpc/grpc-java,stanley-cheung/grpc-java,stanley-cheung/grpc-java,grpc/grpc-java,dapengzhang0/grpc-java,dapengzhang0/grpc-java,dapengzhang0/grpc-java,ejona86/grpc-java,grpc/grpc-java,ejona86/grpc-java,ejona86/grpc-java,stanley-cheung/grpc-java,ejona86/grpc-java,dapengzhang0/grpc-java