Customised InfoWindow of Google Map does not render elevation shadow in Android - google-maps-android-api-2

Question

I'm using InfoWindow to render any information for marker.
In Android, we can define shadow with elevation.
But the shadow defined by elevation is not rendered for InfoWindow.
The layout of infoWindow is like this
<RelativeLayout
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:layout_width="wrap_content"
android:layout_height="wrap_content"
android:background="#drawable/background"
android:paddingStart="16dp"
android:paddingEnd="16dp"
android:paddingTop="6dp"
android:paddingBottom="6dp"
android:elevation="4dp"
>
<TextView
android:id="#+id/main_text"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_centerHorizontal="true"
android:gravity="center_horizontal"
android:textSize="14sp"
android:textStyle="bold"
tools:text="main text"
/>
<TextView
android:id="#+id/sub_text"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="#id/main_text"
android:layout_centerHorizontal="true"
android:gravity="center_horizontal"
android:textSize="11sp"
tools:text="sub text"/>
</RelativeLayout>
Shape is like this
<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
android:shape="rectangle">
<corners android:radius="500dp"/>
<solid android:color="#color/white"/>
</shape>
Code snippet is like this
googleMap?.setInfoWindowAdapter(object: GoogleMap.InfoWindowAdapter {
override fun getInfoContents(marker: Marker?): View? {
return null
}
override fun getInfoWindow(marker: Marker?): View? {
return layoutInflater.inflate(R.layout.map_info_window, null)
}
})