[AlertDialog][a11y] Make dialog icon not important for accessibility

Resolves #1400

PiperOrigin-RevId: 419600919

Author: paulfthomas

lib/java/com/google/android/material/dialog/res/values/styles.xml

@@ -52,7 +52,8 @@
     <item name="android:layout_height">32dip</item>
     <item name="android:scaleType">fitCenter</item>
     <item name="android:src">@null</item>
-    <item name="android:contentDescription">@string/icon_content_description</item>
+    <item name="android:contentDescription">@null</item>
+    <item name="android:importantForAccessibility" tools:targetApi="jelly_bean">no</item>
   </style>
 
   <style name="Base.MaterialAlertDialog.MaterialComponents.Title.Text" parent="RtlOverlay.DialogWindowTitle.AppCompat">