[この質問には自ら回答しました]

react-native で android アプリを起動しようとすると下記のようなエラーが発生します

$ react-native run-android
Starting JS server...
Building and installing the app on the device (cd android && ./gradlew installDebug)...
:app:preBuild UP-TO-DATE
:app:preDebugBuild UP-TO-DATE
:app:checkDebugManifest
.
.
.
:app:zipalignDebug UP-TO-DATE
:app:assembleDebug UP-TO-DATE
:app:installDebug FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':app:installDebug'.
> com.android.builder.testing.api.DeviceException: No connected devices!

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.

BUILD FAILED

Total time: 17.053 secs
Could not install the app on the device, read the error above for details.
Make sure you have an Android emulator running or a device connected and have
set up your Android development environment:
https://facebook.github.io/react-native/docs/android-setup.html

これは何が原因なんでしょうか？