Skip codegen when rebuilding HEAD.

Author: kannanjgithub

buildscripts/kokoro/android.sh

@@ -103,6 +103,7 @@ cd $BASE_DIR/github/grpc-java
 ./gradlew clean
 git checkout HEAD^
 ./gradlew --stop  # use a new daemon to build the previous commit
+GRADLE_FLAGS = $GRADLE_FLAGS + " -PskipCodegen=true"
 ./gradlew publishToMavenLocal $GRADLE_FLAGS
 cd examples/android/helloworld/
 ../../gradlew build $GRADLE_FLAGS