Add content description for preview card of Theme and Grid

Talkback would read "Style preview, double-tap to activate" for theme small preview, and read "Style preview" for theme full preview. And also grid preview.
Video: https://drive.google.com/file/d/11meCX_GTjkDQokJQv9abwfsuGRSL6vpU/view?usp=sharing

Test: Manually
Bug: 157111975
Change-Id: Ieaafad0b6ccc1102bb127f692f6af6b72bc4918a
diff --git a/res/layout/theme_preview_card.xml b/res/layout/theme_preview_card.xml
index 74d1e68..e1e3491 100644
--- a/res/layout/theme_preview_card.xml
+++ b/res/layout/theme_preview_card.xml
@@ -17,6 +17,8 @@
 <androidx.cardview.widget.CardView
     xmlns:android="http://schemas.android.com/apk/res/android"
     style="@style/FullContentPreviewCard"
+    android:id="@+id/theme_preview_card"
+    android:contentDescription="@string/theme_preview_card_content_description"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
     android:layout_gravity="center">
@@ -35,5 +37,6 @@
     <FrameLayout
         android:id="@+id/theme_preview_container"
         android:layout_width="match_parent"
-        android:layout_height="match_parent"/>
+        android:layout_height="match_parent"
+        android:importantForAccessibility="noHideDescendants" />
 </androidx.cardview.widget.CardView>
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