Split up FlexibleKeyItem, re-add support for dummy item if user has no secret keys

OpenKeychain/src/main/res/layout/key_list_dummy.xml

@@ -5,7 +5,8 @@
     android:minHeight="?android:attr/listPreferredItemHeight"
     android:gravity="center_vertical"
     android:orientation="horizontal"
-    android:focusable="true"
+    android:paddingLeft="12dp"
+    android:paddingRight="12dp"
     android:background="?android:selectableItemBackground">
 
         <LinearLayout

OpenKeychain/src/main/res/layout/key_list_item.xml

@@ -8,9 +8,7 @@
     android:gravity="center_vertical"
     android:maxLines="1"
     android:orientation="horizontal"
-    android:descendantFocusability="blocksDescendants"
     android:background="@drawable/list_item_ripple"
-    android:focusable="false"
     android:paddingLeft="12dp"
     android:paddingRight="12dp"
     tools:layout_marginTop="30dp">