How to get a circular shape in a gridview cell in android

I am using gridview to create a simple calculator app. I am using gridView for holding calculator buttons. The gridview is a layout with a TextView. DextView uses a pull-out background that uses an oval shape. Below are the XML views:

Xml grid grid    

    <TextView
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:id="@+id/id_gridCell"
        android:background="@drawable/circularbutton_selector"
        android:layout_margin="0dp"
        android:gravity="center_horizontal|center_vertical"
        android:textSize="40dp"
        android:text="-"
        android:layout_gravity="center_horizontal|center_vertical"
        android:elevation="5dp"
        /> </LinearLayout>

      

        
        
        
      

    

Hand drawn shape

xmlns:android="http://schemas.android.com/apk/res/android" android:shape="oval">

        <solid android:color="@color/accent_material_dark"/>
        <stroke android:width="2dp" android:color="#AAA"/> </shape>

      

        
        
        
      

    

Question: Depending on the width / height of the cell, the shape ends up being oval rather than round. I cannot provide a hard value as the children of the grid must scale to any device. Is there a way to achieve a circle that scales with the dimensions of the device? The idea is to get the scale as the margins change.