[dbsc] Implement garbage collection for unexportable keys in SessionServiceImpl.

This change adds logic to SessionServiceImpl to identify and delete
unexportable keys that are no longer associated with any device-bound
sessions. While loading sessions from the store, the service queries the
unexportable key service for all existing keys. It then compares these
keys against the keys used by the loaded sessions. Any keys found in the
unexportable key service but not referenced by any session are
considered stale and are asynchronously deleted. This garbage collection
process is gated by the kUnexportableKeyDeletion feature flag.

Bug: 455538313
Change-Id: I3b3d55c0ff8cf0efa6452ab08152174690608763
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7212296
Reviewed-by: Daniel Rubery <drubery@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1553303}