Fix mesh when using app-based holographic remoting

[keveleigh]

Overview

Updates our XRSDK-based mesh observers to:

1. Properly refresh their mesh subsystem when XR is initialized after MRTK is initialized
2. Ensure that more than one observer is not running when the "generic" observer is used with a more specific observer (like the one specific to OpenXR)
3. Ensured that we don't try to use a mesh subsystem that isn't actively running. This was leading to
4. Ensured that we don't try to set values on the mesh subsystem unless the app is running (the underlying bug here would probably also be solved by fix 3)

Changes

• Fixes: Spatial Awarness System doesn't work in Holographic Remoting Player v2.7.1 from build #10293, fixes If a hololens 2 goes to sleep while running a Unity OpenXR MRTK app with spatial awareness enabled, the app throws exception and runs poorly after resuming #10244, might fix If the Unity splash screen is disabled for an OpenXR hololens 2 app where spatial awareness is enabled, the spatial mesh is not built. #10245 (but I couldn't get a repro)

[keveleigh]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[keveleigh]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).