Initial ARCore integration

[ryantrem]

This change adds initial support for XR on Android via ARCore.

• Moves XR.cpp (and some other associated files) into an OpenXR specific directory.
• Adds a new XR.cpp that is backed by ARCore (instead of OpenXR).
• Adds ARCore SDK as a submodule, and updates to cmake and gradle files to consume it.
• Minor updates to Playground app to request camera permissions (this is harder to do inside the XR implementation, so I think making this a requirement of the caller for now is ok).
• Adds a Platform class for Android with some platform specific stuff, particularly for getting a JNIEnv attached to the JS thread (@bghgary added this).

This all provides initial basic functionality, but there are a bunch of things that still need to be done:

• Handling surface resizing/screen rotation.
• Handling suspend resume.
• Handling devices that don't support ARCore.
• Checking for ARCore package installation on the device (this might be another thing that the consuming app is required to do, not sure yet).
• Support for plane detection, hit testing, and anchors (needs to be plumbed through more layers because this is not supported in the pre-existing OpenXR implementation of XR.h).

[bghgary]

should we fix this before merge?

[ryantrem]

My vote is no. I want to stop having to merge cmakelists.txt files :)

[bghgary]

Okay, maybe we can file an issue on it.

[bghgary]

why do you need this?

[ryantrem]

I'm using gsl::finally in xr.cpp to revert OpenGL state changes I make since we are taking over the gl context every frame (sharing it with Babylon rendering). Ideally we would use our own context that wraps the context used by Babylon rendering, but I never was able to get this to work.

[bghgary]

Okay. I'm a bit concerned about reaching into a dependency's dependency. Not sure what the implications are.