Make XR begin and end session asynchronous

[syntheticmagus]

Work will require the XrPlugin to have access to the task chain, so should be undertaken after the work to switch NativeEngine to us runtime global state stored with NAPI so that XrPlugin can take advantage of the same mechanism.

[ryantrem]

Begin session was made async as part of 45a1944.

[bghgary]

Closing for now. Reopen if we want to fix this.