The button is called a Floating action button or "FAB". What you want is a FAB on a CoordinatorLayout. For further reading you can go here:
You should look for "FloatingActionButton".
For example:
<android.support.design.widget.CoordinatorLayout xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="match_parent">
<LinearLayout
android:id="@+id/viewA"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_weight="0.15">
</LinearLayout>
<LinearLayout
android:id="@+id/viewB"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_weight="0.85" >
</LinearLayout>
</LinearLayout>
<android.support.design.widget.FloatingActionButton
android:id="@+id/btn_add"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_margin="@dimen/activity_horizontal_margin"
android:clickable="true"
android:src="@drawable/ic_ok"
app:layout_anchor="@id/viewA"
app:layout_anchorGravity="left|bottom"
app:backgroundTint="@color/accent"/>
</android.support.design.widget.CoordinatorLayout>
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