Optimize map lookups and avoid unnecessary copies in BocaSessionManager.

Refactors `GetStudentActiveDeviceId` and `NotifyConsumerActivityUpdated`
to use `find()` instead of `contains()` and `at()` for efficiency. Also,
uses references to avoid copying large map objects.

Change-Id: I6b65e09236f70d2f07c7ca10cd0b9ef834f2a6f1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7038693
Commit-Queue: Jeffrey Yu <yuje@google.com>
Reviewed-by: April Zhou <aprilzhou@google.com>
Reviewed-by: Benjamin Zielinski <bzielinski@google.com>
Cr-Commit-Position: refs/heads/main@{#1529179}