Migrate last batch of XR snippets

[devbridie]

Migrate the following pages to snippets:

• https://developer.android.com/develop/xr/jetpack-xr-sdk/transition-home-space-to-full-space
• https://developer.android.com/develop/xr/jetpack-xr-sdk/check-spatial-capabilities
• https://developer.android.com/develop/xr/jetpack-xr-sdk/add-3d-models

[snippet-bot]

Here is the summary of changes.

You are about to add 8 region tags.

• xr/src/main/java/com/example/xr/compose/SpatialCapabilities.kt:50, tag androidxr_compose_checkSpatialUiEnabled
• xr/src/main/java/com/example/xr/misc/ModeTransition.kt:25, tag androidxr_misc_modeTransitionCompose
• xr/src/main/java/com/example/xr/misc/ModeTransition.kt:33, tag androidxr_misc_modeTransitionScenecore
• xr/src/main/java/com/example/xr/scenecore/GltfEntity.kt:30, tag androidxr_scenecore_gltfmodel_create
• xr/src/main/java/com/example/xr/scenecore/GltfEntity.kt:36, tag androidxr_scenecore_gltfmodelentity_create
• xr/src/main/java/com/example/xr/scenecore/GltfEntity.kt:46, tag androidxr_scenecore_gltfmodelentity_animation
• xr/src/main/java/com/example/xr/scenecore/GltfEntity.kt:52, tag androidxr_scenecore_sceneviewer
• xr/src/main/java/com/example/xr/scenecore/SpatialCapabilities.kt:24, tag androidxr_compose_checkMultipleCapabilities

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