Use material icon for safeslinger

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

@@ -58,7 +58,7 @@
             android:layout_width="wrap_content"
             android:layout_height="match_parent"
             android:layout_gravity="center"
-            android:src="@drawable/ic_action_safeslinger"
+            android:src="@drawable/ic_swap_vert_grey_24dp"
             android:padding="12dp"
             style="@style/SelectableItem" />
 

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

@@ -69,7 +69,7 @@
                     android:layout_width="wrap_content"
                     android:layout_height="match_parent"
                     android:padding="8dp"
-                    android:src="@drawable/ic_action_safeslinger"
+                    android:src="@drawable/ic_swap_vert_grey_24dp"
                     android:layout_gravity="center_vertical" />
 
             </LinearLayout>

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

@@ -135,7 +135,7 @@
                 android:layout_width="wrap_content"
                 android:layout_height="match_parent"
                 android:padding="8dp"
-                android:src="@drawable/ic_action_safeslinger"
+                android:src="@drawable/ic_swap_vert_grey_24dp"
                 android:layout_gravity="center_vertical"
                 style="@style/SelectableItem" />