How to disable landscape mode in React Native Android dev mode?

Add android:screenOrientation="portrait" to the activity section in android/app/src/main/AndroidManifest.xml file, so that it end up looking like this:

<activity
    android:name=".Activity"
    android:label="Activity"
    android:screenOrientation="portrait"
    android:configChanges="keyboardHidden|orientation|screenSize">
</activity>

There are several different values for the android:screenOrientation property; for a comprehensive list take a look at the following: https://developer.android.com/guide/topics/manifest/activity-element.html