Gvr-unity-sdk: Need option to remove alignment marker, back button and settings button

[MichaelPote]

This has been an ongoing issue since 0.8.5 with repo issues #550 and #266 and numerous stack overflow and Unity forms questions showing that many, many developers want these options back.

I've just had to delete my project using Gvr 1.190 and recreate the entire unity project using the 0.8 sdk just so that I can disable these buttons.

I cannot fathom how removing the option to hide these is a good idea or the team's blank refusal to even consider reinstating it.

In my scenario, the cardboard viewers we're using allow the top two corners of the phone to stick out of the cardboard viewer and users naturally brush these corners and touch them. Guess what GVR sticks in the corners of the screen? Yes. the settings and back buttons. So every time a user picks up one of our cardboard devices, the settings menu opens. Brilliant.

[rusmaxham]

We will look into what we can do to provide more flexibility of the GVR supplied UX layer.