When snapshots are disabled, fill it with single color.

Test: Launch DisableScreenshotsActivity, go to recents, make sure
content is blue. Reopen activity from home, make sure starting
window is blue.

Bug: 31339431
Change-Id: I29689774c3cdcb784d8f5bfa4f947a6f35b91e01
diff --git a/tests/ActivityTests/AndroidManifest.xml b/tests/ActivityTests/AndroidManifest.xml
index 4dd51dc..64cdcf7 100644
--- a/tests/ActivityTests/AndroidManifest.xml
+++ b/tests/ActivityTests/AndroidManifest.xml
@@ -80,7 +80,8 @@
         <receiver android:name="TrackTimeReceiver" />
         <receiver android:name="AlarmSpamReceiver" />
         <activity android:name="DisableScreenshotsActivity"
-                android:label="DisableScreenshots">
+                android:label="DisableScreenshots"
+                android:theme="@style/DisableScreenshots">
             <intent-filter>
                 <action android:name="android.intent.action.MAIN" />
                 <category android:name="android.intent.category.LAUNCHER" />