Darken background of feed header

This change only applies to feed items using transparent backgrounds.
The item is darkened but the background keeps transparent, which looks
weird, especially in light theme (have a look at the 34C3 feed)

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

@@ -4,6 +4,7 @@
     xmlns:tools="http://schemas.android.com/tools"
     android:layout_width="match_parent"
     android:layout_height="wrap_content"
+    android:background="#5000"
     tools:context="de.danoeh.antennapod.activity.MainActivity"
     tools:background="@android:color/darker_gray">