Add infrastructure for androidx.xr and snippets for ARCore for Jetpack XR Hands

[devbridie]

This PR adds infrastructure for the Android XR team and ports one page to this repo.

[snippet-bot]

Here is the summary of changes.

You are about to add 3 region tags.

• xr/src/main/java/com/example/xr/arcore/Hands.kt:73, tag androidxr_arcore_hand_collect
• xr/src/main/java/com/example/xr/arcore/Hands.kt:91, tag androidxr_arcore_hand_entityAtHandPalm
• xr/src/main/java/com/example/xr/arcore/Hands.kt:112, tag androidxr_arcore_hand_entityAtIndexFingerTip

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[devbridie]

Regarding the snippet-bot finding, please see cl/728729245 which adds the product prefix. Since this CL was submitted recently, it will take until wednesday for it to appear in prod.