I would like to align this `ImageView` in `center` (horizontal and vertical): 

```
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/titletab"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="@dimen/slidingtab_icon_header"
    android:gravity="center">

    <ImageView
        android:id="@+id/icon"
        android:layout_width="@dimen/slidingtab_item_icon_size"
        android:layout_height="@dimen/slidingtab_item_icon_size"
        android:layout_gravity="center_horizontal"
        android:src="@drawable/ic_event_black_18dp"/>

    <View
        android:layout_width="match_parent"
        android:layout_height="1dp"
        android:layout_alignParentBottom="true"
        android:background="@color/sliding_tab_border_color"/>

</RelativeLayout>

```

If I don't add the line (the `View`) after my `ImageView` it's perfect, it's work, but with this line the `gravity` doesn't work.

Thanks by advance!