Signal-Android/res/drawable/conversation_item_sent_triangle_shape.xml

<?xml version="1.0" encoding="utf-8"?>
<layer-list xmlns:android="http://schemas.android.com/apk/res/android" >
    <item>
        <rotate
            android:fromDegrees="45"
            android:toDegrees="45"
            android:pivotX="0%"
            android:pivotY="-30%" >
            <shape
                android:shape="rectangle" >
                <solid
                    android:color="@color/conversation_item_sent_background_light" />
            </shape>
        </rotate>
    </item>
</layer-list>