Button for API < 21

shape_blue_button.xml

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

    <item android:state_enabled="true" android:state_pressed="true">
        <layer-list>
            <!-- button background shadow -->
            <item android:left="2dp" android:top="2dp">
                <shape android:shape="rectangle">
                    <solid android:color="@color/blue_shadow" />
                    <corners android:radius="6dp" />
                    <padding android:bottom="1.5dp" android:right="1.5dp" />
                </shape>
            </item>
            <!-- button -->
            <item android:left="1dp" android:top="1dp">
                <shape>
                    <solid android:color="@color/blue_pressed" />
                    <corners android:radius="6dp" />
                </shape>
            </item>
        </layer-list>
    </item>

    <item>
        <layer-list>
            <!-- button background shadow -->
            <item android:left="2dp" android:top="2dp">
                <shape android:shape="rectangle">
                    <solid android:color="@color/blue_shadow" />
                    <corners android:radius="6dp" />
                    <padding android:bottom="2dp" android:right="2dp" />
                </shape>
            </item>
            <!-- button -->
            <item>
                <shape>
                    <solid android:color="@color/blue" />
                    <corners android:radius="6dp" />
                </shape>
            </item>
        </layer-list>
    </item>

</selector>