[Vitreous] Implement OpenXR entity and activity space

Implements OpenXrEntity, OpenXrSystemSpaceEntity, and
OpenXrActivitySpace in scenecore-openxr, handling coordinate
transformations across Space.PARENT, Space.ACTIVITY, and
Space.REAL_WORLD, bounds change listeners, and parent hierarchy
tracking. Wires OpenXrActivitySpace, PerceptionSpaceScenePoseImpl,
and entity creation into OpenXrSceneRuntime.

Bug: 538933751
Test: ./gradlew :xr:scenecore:scenecore-openxr:testReleaseUnitTest
Change-Id: Iefce9f0d60f718781c2e339c1cf9879240c62579