Fix flashing when entering text story.

app/src/main/res/layout/stories_text_post_preview_fragment.xml

@@ -12,5 +12,6 @@
         android:id="@+id/story_text_post_shared_element_target"
         android:layout_width="match_parent"
         android:layout_height="match_parent"
-        android:importantForAccessibility="no" />
+        android:importantForAccessibility="no"
+        android:src="@color/core_grey_80" />
 </FrameLayout>