Is there a way to show a preview of a RecyclerView's contents as Grid in the Android Studio editor?

You are able to build a preview using xmlns:tools="http://schemas.android.com/tools" namespace.

AndroidX^[About]

<androidx.recyclerview.widget.RecyclerView
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"

    android:layout_width="match_parent"
    android:layout_height="match_parent"

    tools:layoutManager="androidx.recyclerview.widget.GridLayoutManager"
    tools:listitem="@layout/item"
    tools:itemCount="7"
    tools:orientation="vertical"
    tools:scrollbars="vertical"
    tools:spanCount="4"/>

Support

<android.support.v7.widget.RecyclerView
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"

    android:layout_width="match_parent"
    android:layout_height="match_parent"

    tools:layoutManager="android.support.v7.widget.GridLayoutManager"
    tools:listitem="@layout/item"
    tools:itemCount="7"
    tools:orientation="vertical"
    tools:scrollbars="vertical"
    tools:spanCount="4"/>

From Android studio 3.0 you can predefine a data through the @tools:sample/* resources

item.xml

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="100dp"
    android:layout_height="150dp"
    android:layout_marginBottom="15dp"
    android:orientation="vertical"
    tools:background="@tools:sample/avatars">

    <TextView
        android:layout_gravity="bottom"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textColor="@color/colorWhite"
        tools:text="@tools:sample/first_names" />

</FrameLayout>

As a result your preview will look like

[More examples]

If you want to see the effect only in the preview without change the app behavior you can use the "tools" namespace as you did with listitem:

<android.support.v7.widget.RecyclerView
        android:id="@+id/rcv_collection"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:layoutManager="android.support.v7.widget.GridLayoutManager"
        tools:spanCount="2"
        tools:listitem="@layout/item_collection"/>

This Worked for me (AndroidX): For 3 column grid

<androidx.recyclerview.widget.RecyclerView
    android:id="@+id/rvServiceList"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    app:layoutManager="androidx.recyclerview.widget.GridLayoutManager"
    app:spanCount="3"
    tools:itemCount="20"
    tools:listitem="@layout/row_item_service" />