Prevent GCM.cs from failing Desktop builds.
Prevent GCM.cs from failing Desktop builds.

Added "#if UNITY_ANDROID" compile time instructions to prevent the GCM.cs file from throwing errors when building desktop versions of a Unity game. An error sample is provided for context:
countly Assets/Plugins/GCM/GCM.cs(28,32): error CS0246: The type or namespace name `AndroidJavaClass' could not be found.
ramyD/countly-sdk-unity,ramyD/countly-sdk-unity,ramyD/countly-sdk-unity