Dialer: Apply rounded edges to all AlertDialogs

Turns out, we need to set android:windowBackground, not just
android:background or any of the usual suspects

Change-Id: I3bfdc2714d8450554a48452dba92a1abaf6a346e
diff --git a/java/com/android/dialer/theme/hidden/res/values/styles.xml b/java/com/android/dialer/theme/hidden/res/values/styles.xml
index dbd9ab9..b2fab95 100644
--- a/java/com/android/dialer/theme/hidden/res/values/styles.xml
+++ b/java/com/android/dialer/theme/hidden/res/values/styles.xml
@@ -55,5 +55,6 @@
     <item name="android:colorAccent">@color/dialer_theme_color</item>
     <item name="colorAccent">@color/dialer_theme_color</item>
     <item name="android:textColorSecondary">@color/dialer_secondary_text_color</item>
+    <item name="android:windowBackground">@drawable/dialog_background</item>
   </style>
 </resources>