Lock orientation to portrait.

app/src/main/AndroidManifest.xml

@@ -7,7 +7,8 @@
             android:label="@string/app_name"
             android:supportsRtl="true"
             android:theme="@style/AppTheme">
-        <activity android:name=".MainActivity">
+        <activity android:name=".MainActivity"
+                  android:screenOrientation="portrait">
             <intent-filter>
                 <action android:name="android.intent.action.MAIN"/>