Own Button shape in Android / using multiple shapes as background

I am working on Android project. I have ListView with custom buttons with rounded shape and white color. What I am trying to add is custom grey shape - rectangle combined with triangle:

Main Button Definition in XML

<Button xmlns:android="http://ift.tt/nIICcg"
    android:id="@+id/button_dropdown_category"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:padding="10dp"
    android:background="@drawable/list_category_button"
    android:minHeight="45dp"
    android:maxHeight="45dp"
    android:layout_marginTop="15dp"
    android:layout_marginBottom="15dp"
    android:layout_marginLeft="15dp"
    android:layout_marginRight="15dp"
    android:paddingLeft="20dp"
    android:text="@null"
    android:textSize="18sp"
    android:gravity="left|center_vertical" />

Shape Button Definition in XML (list_category_button)

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://ift.tt/nIICcg">
<item>
<layer-list>
  <item>
    <shape xmlns:android="http://ift.tt/nIICcg"
           android:shape="rectangle"
           >
      <solid android:color="@color/main_red_color" />
      <size android:height="50dp"
    android:width="50dp"/>
    </shape>
  </item>
  <item>
    <shape>
      <solid android:color="@color/main_white_color" />
      <stroke
          android:width="1dp"
          android:color="@color/main_darkgrey_color" />
      <corners
          android:radius="4dp" />
      <padding
          android:left="3dp"
          android:top="3dp"
          android:right="3dp"
          android:bottom="3dp" />
    </shape>
  </item>
</layer-list>
</item>
</selector>