add context menu to autocrypt identity items

OpenKeychain/src/main/res/menu/identity_context_menu.xml

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="utf-8"?>
+<menu xmlns:android="http://schemas.android.com/apk/res/android">
+
+    <item
+        android:id="@+id/autocrypt_forget"
+        android:title="@string/identity_context_forget"
+        android:icon="@drawable/ic_delete_grey_24dp"
+        />
+
+</menu>