Have you tried this?
android:screenOrientation="portrait"
Inside the AndroidManifest.xml
file, where you have declared your activity, just add the above line.
For example:
<activity
android:name=".Activity.SplashScreenActivity"
android:label="@string/app_name"
android:screenOrientation="portrait">
<intent-filter>
<action
android:name="android.intent.action.MAIN" />
<category
android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>