Disable starting window for Recents

When using a starting window, transition could start early before
real app window is drawn. We don't want this with Recents, so disable
it explicitly.

bug: 27812497
Change-Id: I648b2de3f91923978ef8b3d4f2629f0f73321b98

packages/SystemUI/res/values/styles.xml

@@ -33,6 +33,7 @@
         <item name="android:windowBackground">@color/transparent</item>
         <item name="android:colorBackgroundCacheHint">@null</item>
         <item name="android:windowShowWallpaper">true</item>
+        <item name="android:windowDisablePreview">true</item>
     </style>
 
     <!-- Performance optimized Recents theme (no wallpaper) -->