Half circle shape not work

you can try this :

<?xml version="1.0" encoding="utf-8"?>
<shape
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="rectangle">
    <solid android:color="#900021df"/>
    <size
        android:width="10dp"
        android:height="5dp"/>
    <corners
        android:bottomLeftRadius="20dp"
        android:bottomRightRadius="20dp"/>
</shape>

it gives this shape:

You cannot create a semicircle from xml. but you could achieve what you are looking for using a circle with appropriate margin & padding.

You can use a circle shape .xml file. Create a fixed sized circle like this:

Example:

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
       android:shape="oval"
       android:useLevel="false" >
   <solid android:color="#006AC5" />
   <size
    android:height="50dp"
    android:width="50dp" />
</shape>

curve_toolbar_bg.xml

<?xml version="1.0" encoding="utf-8"?>
<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
    <item>
        <shape android:shape="rectangle"/>
    </item>
    <item
        android:bottom="0dp"
        android:left="-100dp"
        android:right="-100dp"
        android:top="-80dp">
        <shape android:shape="oval">
            <solid android:color="@color/colorPrimary" />
        </shape>
    </item>
</layer-list>

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <android.support.v7.widget.Toolbar
        android:id="@+id/toolbar"
        android:layout_width="0dp"
        android:layout_height="?android:attr/actionBarSize"
        android:background="@drawable/rounded_corner"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.0">

    </android.support.v7.widget.Toolbar>
</android.support.constraint.ConstraintLayout>