Add information on accessing the underlying mobile runtime

[devbridie]

Adds dependencies and an example on how to access an underlying ARCore mobile runtime on ARCore for Jetpack XR.

[snippet-bot]

Here is the summary of changes.

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• xr/src/main/java/com/example/xr/arcore/Mobile.kt:25, tag androidxr_arcore_mobile_runtime

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[amyZepp]

LGTM!