Android : I need to know how to set a button at the very bottom of the background

I'm implementing a simple application for my project. I applied a background image for the layout and i need to have a button to control layouts. Once I apply button at the very bottom it is not moving out of the layout. Please help me to sort this out!

And this is my xml code

  <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
      xmlns:tools="http://schemas.android.com/tools"               android:layout_width="match_parent"
      android:layout_height="match_parent"      android:paddingLeft="@dimen/activity_horizontal_margin"
     android:paddingRight="@dimen/activity_horizontal_margin"
     android:paddingTop="@dimen/activity_vertical_margin"
     android:paddingBottom="@dimen/activity_vertical_margin"
     tools:context="com.example.design.a.Chats"
     android:background="@drawable/chatpage"

    >

      <Button
    style="?android:attr/buttonStyleSmall"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Chat"
    android:id="@+id/btnChats"
    android:clickable="false"
    android:layout_alignParentBottom="true"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true" />
  </RelativeLayout>