tls-psk: add compatibility fragment for android < 5

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

@@ -0,0 +1,27 @@
+<?xml version="1.0" encoding="utf-8"?>
+<LinearLayout
+    xmlns:android="http://schemas.android.com/apk/res/android"
+    android:layout_width="match_parent"
+    android:layout_height="match_parent"
+    android:gravity="center_vertical"
+    android:orientation="vertical"
+    android:padding="16dp">
+
+    <ImageView
+        android:layout_width="96dp"
+        android:layout_height="96dp"
+        android:layout_gravity="center_horizontal"
+        android:layout_marginBottom="48dp"
+        android:src="@drawable/ic_android_96dp"
+        android:tint="@color/md_grey_400"
+        />
+
+    <TextView
+        android:layout_width="wrap_content"
+        android:layout_height="wrap_content"
+        android:layout_gravity="center_horizontal"
+        android:layout_margin="8dp"
+        android:text="@string/transfer_not_available"
+        android:textAppearance="?android:attr/textAppearanceMedium" />
+
+</LinearLayout>

OpenKeychain/src/main/res/values/strings.xml

@@ -1901,5 +1901,6 @@
     <string name="transfer_error_read_incoming">"Failed reading incoming key!"</string>
     <string name="transfer_error_connect">"Connection failed!"</string>
     <string name="transfer_error_listen">"Error waiting for connection!"</string>
+    <string name="transfer_not_available">"Sorry, this feature can only be used on Android 5 or newer :("</string>
 
 </resources>