fix(Controller): ensure cached tracked object index does not exist
fix(Controller): ensure cached tracked object index does not exist

An additional error could occur where the tracked object index
would already exist in the cache, so a simple check is done before
caching it to make sure it's not in the index.

virror/VRTK,nmccarty/htvr,gomez-addams/SteamVR_Unity_Toolkit,red-horizon/VRTK,Fulby/VRTK,gpvigano/VRTK,mattboy64/SteamVR_Unity_Toolkit,adamjweaver/VRTK,pargee/SteamVR_Unity_Toolkit,Blueteak/SteamVR_Unity_Toolkit,Innoactive/IA-unity-VR-toolkit-VRTK,gpvigano/SteamVR_Unity_Toolkit,thestonefox/VRTK,thestonefox/SteamVR_Unity_Toolkit,TMaloteaux/SteamVR_Unity_Toolkit