add subtle attention seeker to decryptactivity with available clipboard data

Thanks Cyril Mottier

https://plus.google.com/+CyrilMottier/posts/FABaJhRMCuy

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

@@ -88,6 +88,7 @@
             </LinearLayout>
 
             <ImageView
+                android:id="@+id/clipboard_icon"
                 android:layout_width="wrap_content"
                 android:layout_height="match_parent"
                 android:padding="8dp"