In your root project directory
Make sure if you have already directory android/app/src/main/assets/ if not there create directory after that create new file and save as index.android.bundle and put your file in like this android/app/src/main/assets/index.android.bundle
After run this in cmd
react-native bundle --platform android --dev false --entry-file index.js --bundle-output android/app/src/main/assets/index.android.bundle --assets-dest android/app/src/main/res/
cd android && ./gradlew assembleDebug
then you can get apk app/build/outputs/apk/debug/app-debug.apk