Currently, we use the presence of the DOWNLOAD_VR_TEST_APKS environment variable to determine whether we should download VR and AR test APKs. This should be renamed to DOWNLOAD_XR_TEST_APKS since it's now used for more than just VR.

This requires updating all the places in src/ that use it, as well as the definition (https://cs.chromium.org/chromium/build/scripts/slave/recipe_modules/chromium/config.py?q=chromium/config.py&sq=package:chromium&dr&l=800) and its uses in infra-side code.