passdialog: seconds field and some minor layout issues

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

@@ -39,7 +39,7 @@
             android:layout_height="wrap_content"
             android:layout_gravity="center"
             android:gravity="center_vertical"
-            android:layout_margin="6dp">
+            android:layout_margin="8dp">
 
             <TextView
                 android:layout_width="wrap_content"

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

@@ -5,12 +5,13 @@
     android:orientation="vertical"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
-    android:gravity="center">
+    android:gravity="center_vertical">
 
     <TextView
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:id="@+id/simple_item_text"
         android:textAppearance="@android:style/TextAppearance.Medium"
+        android:padding="8dp"
         tools:text="itemtext" />
 </LinearLayout>