Add UMAs for scene understanding manager usage

Adds two new UMAs. One will inform us how often each scene understanding
manager is actually created, or fails to be created. This will help
understand overall availability/fit of the managers as well as highlight
gaps where the current set of scene understanding managers may not be
sufficient. The second will inform us how commonly supported the MSFT
extension based Scene Understanding manager is, regardless of if it were
created as the best choice or not. This will be used to evaluate if we
can hopefully consolidate down to just the spatial entities scene
understanding manager, since it is supported by XR_EXT extensions rather
than vendor-specific extensions.

Bug: 394772465
Change-Id: I83a6c8b12443c025250dd0d7bf938fbf3f55155e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6944564
Commit-Queue: Alexander Cooper <alcooper@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1515132}