Rounded button background with 1dp border

btn_circle_bg.xml

<?xml version="1.0" encoding="utf-8"?>

<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:drawable="@drawable/btn_digit_circle_pressed_bg" android:state_pressed="true" />
    <!-- pressed -->

    <item android:drawable="@drawable/btn_digit_circle_default_bg" />
    <!-- default -->

</selector>

btn_circle_default_bg.xml

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="oval">

    <stroke
        android:width="1dp"
        android:color="#ffffff" />
    <solid android:color="#00000000" />

    <size
        android:width="48dp"
        android:height="48dp" />
</shape>

btn_circle_pressed_bg.xml

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="oval">

    <stroke
        android:width="1dp"
        android:color="#ffffff" />
    <solid android:color="#88ffffff" />

    <size
        android:width="48dp"
        android:height="48dp" />
</shape>