session-android/res/xml/pin_keyboard.xml

<?xml version="1.0" encoding="utf-8"?>
<Keyboard xmlns:android="http://schemas.android.com/apk/res/android"
        android:keyWidth="33.3%p"
        android:keyHeight="10%p">

    <Row>
        <Key android:codes="1"    android:keyLabel="1"  android:keyEdgeFlags="left" />
        <Key android:codes="2"    android:keyLabel="2" />
        <Key android:codes="3"    android:keyLabel="3" />
    </Row>

    <Row>
        <Key android:codes="4"    android:keyLabel="4" android:keyEdgeFlags="left"  />
        <Key android:codes="5"    android:keyLabel="5" />
        <Key android:codes="6"    android:keyLabel="6" />
    </Row>

    <Row>
        <Key android:codes="7"    android:keyLabel="7" android:keyEdgeFlags="left" />
        <Key android:codes="8"    android:keyLabel="8" />
        <Key android:codes="9"    android:keyLabel="9" />
    </Row>
    <Row>
        <Key android:codes="0"    android:keyLabel="0" android:horizontalGap="33.3%p"/>
        <Key android:codes="-1"    android:keyIcon="@drawable/ic_backspace_grey600_24dp" android:isRepeatable="true"/>
    </Row>
</Keyboard>