Use the same fragment id for portrait & landscape

To refresh the header, we retrieve the fragment from its id.

But the landscape layout used another id for the same fragment, so it
could not be retrieved, leading to a NullPointerException in landscape.

Therefore, use the same fragment id as in the portrait layout.

F-Droid/res/layout-land/app_details.xml

@@ -22,7 +22,7 @@
             <fragment
                 android:layout_width="fill_parent"
                 android:layout_height="wrap_content"
-                android:id="@+id/landscape_header"
+                android:id="@+id/header"
                 android:name="org.fdroid.fdroid.AppDetails$AppDetailsHeaderFragment"
                 tools:layout="@layout/app_details_header"/>