improve info messages for dark theme

this improves readability of info messages
by setting the foreground to while.
`universal_overlay` was made for that;
it is in use like that on iOS/desktop since a lont time.

moreover, other parts using the combination of
`conversation_item_update_bg_color`/`conversation_item_update_text_color`
are improved equally, eg. the day titles.

Author: r10s

src/main/res/values/themes.xml

@@ -189,7 +189,7 @@
         <item name="conversation_item_outgoing_text_secondary_color">@color/light_pink</item>
 
         <item name="conversation_item_update_bg_color">@color/universal_overlay</item>
-        <item name="conversation_item_update_text_color">@color/core_light_35</item>
+        <item name="conversation_item_update_text_color">@color/white</item>
 
         <item name="conversation_item_sent_indicator_text_background">@drawable/conversation_item_sent_indicator_text_shape_dark</item>
         <item name="conversation_item_date_line_color">@color/core_dark_55</item>