Elaborating on @alexander 's answer, you can use the following workflow:
cd android
./gradlew assembleDebug # assembleRelease for release builds
adb install -s <yourdevice> app/build/outputs/apk/yourapk.apk # You can check the identifier for your device with adb devices
adb reverse tcp:8081 tcp:8081 Will forward the phone's 8081 port to the computer's 8081, where the packager listens. If you are deploying over Wi-Fi, have a look at: https://facebook.github.io/react-native/docs/running-on-device-android.html#configure-your-app-to-connect-to-the-local-dev-server-via-wi-fi
In a different screen, run:
npm start # Will run the packager
If you really need this, you might want to wrap the first snippet into a script that you can parametrize with your phone's identifier.
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