Force GenerateImagesActivity to portrait orientation.

We landed a fix that force ThemeDeviceActivity to be in portrait orientation (ag/1138692)
however I missed to also force GenerateImagesActivity to the same orientation.

Bug: 31681138
Test: ran this test while rotating the device a bunch of times. It fails without this patch.
Change-Id: Ie1c4028389ad43b0ea2dcde063441a667506e0e4
diff --git a/hostsidetests/theme/app/AndroidManifest.xml b/hostsidetests/theme/app/AndroidManifest.xml
index d39f1ae..d9a89c6 100755
--- a/hostsidetests/theme/app/AndroidManifest.xml
+++ b/hostsidetests/theme/app/AndroidManifest.xml
@@ -31,7 +31,7 @@
                 <category android:name="android.intent.category.LAUNCHER" />
             </intent-filter>
         </activity>
-        <activity android:name=".GenerateImagesActivity"
+        <activity android:name=".GenerateImagesActivity" android:screenOrientation="portrait"
                   android:exported="true" />
     </application>