Use spinner DatePicker, not full calendar, also on Android 5

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

@@ -1,5 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
+    xmlns:tools="http://schemas.android.com/tools"
     android:orientation="vertical"
     android:paddingTop="16dp"
     android:paddingBottom="16dp"
@@ -15,13 +16,23 @@
         android:layout_height="wrap_content"
         android:text="@string/btn_no_date" />
 
+    <!--
+        Use Spinner style DatePicker, not the full calendar view
+        Android < 5:
+            android:spinnersShown="true"
+            android:calendarViewShown="false"
+        Android >= 5:
+            android:datePickerMode="spinner"
+    -->
+    <!-- Hide calendarView in tablets because of the unix warparound bug. -->
     <DatePicker
         android:id="@+id/edit_subkey_expiry_date_picker"
         android:layout_gravity="center_horizontal"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
+        tools:ignore="UnusedAttribute"
+        android:datePickerMode="spinner"
         android:spinnersShown="true"
         android:calendarViewShown="false" />
-    <!-- Hide calendarView in tablets because of the unix warparound bug. -->
 
 </LinearLayout>