usd procedural does not bind materials to instances

**Describe the bug**
When I try to load a layered USD file with materials and material assingments which looks like this:

```
#usda 1.0
(
endTimeCode = 100
framesPerSecond = 24
metersPerUnit = 1
startTimeCode = 1
subLayers= [
@./domelight1.usd@,
@./layerTest_materialassing.usda@,
@./layerTest_materiallib.usda@,
@./layerTest_boxes.usda@
]
timeCodesPerSecond = 24
upAxis = "Y"
)
```

In Maya with MtoA 5.3.1, the light works, the boxes are animated, but the shaders are not applied.
Same file renders fine in Houdini with Arnold/Solaris.

**To Reproduce**
[layers (1).zip](https://github.com/Autodesk/arnold-usd/files/11442017/layers.1.zip)

**Expected behavior**
A clear and concise description of what you expected to happen.

**Screenshots**

Unpack the attached layers (1).zip and reproduce the layered USD case in Maya with MtoA 5.3.1, then compare it with the working Houdini Arnold/Solaris result. Trace the material assignments on instances in the layered USD input; done means the Maya render applies the shaders to the boxes as expected.

Written by the indexing model from the issue text.