Toolbar Elevation on Pre-lollipop Devices.

toolbar.xml

<!-- Create a new XML file toolbar.xml in directory res/layouts --> 

<LinearLayout 
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:orientation="vertical">

<android.support.v7.widget.Toolbar
    android:layout_width="match_parent"
    android:layout_height="?attr/actionBarSize"
    android:background="?attr/colorPrimary" />

<View
    android:layout_width="match_parent"
    android:layout_height="4dp"
    android:background="@drawable/dropshadow" />

</LinearLayout>

toolbar_shadow.xml

<!-- Create a new XML file dropshadow.xml in directory res/drawable -->   

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android" android:shape="rectangle">
    <gradient
        android:startColor="@android:color/transparent"
        android:endColor="#44000000"
        android:angle="90"/>
</shape>