Testing sample for AndroidXR

I have checked Jetpack XR's example and some source code that will be merged into AOSP, and looks like they still use libraries for integration testing like normal Android apps. What about adding a new example sample to demonstrate how to use existing testing library for AndroidXR, the new platform. cc @brettchabot .

Start by reviewing the Jetpack XR example and the AndroidXR source planned for AOSP, then compare how existing integration-testing libraries are used in Android samples. Define the sample's scope and verify that it demonstrates testing AndroidXR with the existing library; no specific files or tests are named in the issue.

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