Remove the announcement of accessibility event for PaletteListView.

Root cause: The color block of color correction have no any content
desciption and it also can't be tapped to trigger others behavior.
Goal: Remove accessibility event

Bug: 148957372
Test: manual test
Change-Id: I97425e24a54e983a49189389746540ed98af82f9

res/layout/daltonizer_preview.xml

@@ -24,6 +24,6 @@
         android:id="@+id/palette_listView"
         android:layout_width="match_parent"
         android:layout_height="wrap_content"
-        android:contentDescription="@null" />
+        android:importantForAccessibility="noHideDescendants" />
 
 </FrameLayout>