SpectatorView prevents Standalone (and any other platform where Handheld doesn't exist) builds

[keveleigh]

Overview

The UnityEngine.Handheld class doesn't exist when building for Standalone (it probably doesn't exist in some number of other non-handheld platforms as well).

Expected Behavior

MRTK doesn't prevent building for whichever platform I want

Actual Behavior

MRTK prevents building for (at least) Standalone

Steps to reproduce

1. Switch the build type to Standalone.
2. Try to build.

Unity Editor Version

Any

Mixed Reality Toolkit Release Version

2017.4.0.0