[sessions] Route writeable UTC config cap to session.

This will be used by Starnix containers running in the session.

Change-Id: I28acce7644c9cce9a84b3feaf1358967d8ee8bd6
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1236465
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>
Reviewed-by: Joshua Liebow-Feeser <joshlf@google.com>
Fuchsia-Auto-Submit: Adam Perry <adamperry@google.com>
Reviewed-by: Caroline Liu <carolineliu@google.com>