boca: Provide a channel to subscribe local only session(caption) update.

In order to make this happen, a few changes included:
1. Provide a way to register BocaSessionManager for it's current
   profile, so that /ash/webui/ can access the respective instance
   without involving browser dependency.

2. Update the ownership for session_client_impl in boca_session_manager,
   as it should have been instantiated at a higher level and pass
   pointer only.

3. Replace the way for fetching identity from Ash Shell to User Manager so that the unit test doesn't have to depend on Aura instance.

Test: Unit tested.
Bug: b:362291997
Low-Coverage-Reason: HARD_TO_TEST in boca_app_client.cc the identity manager implementation resides in c/b/ash, it's hard to cross the path and unit test. But the behavior is tested in consumer of this file.
Change-Id: I50048509b9a1a4ddaa71439db63ed7d37c306717
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5815051
Reviewed-by: Alex Ilin <alexilin@chromium.org>
Commit-Queue: April Zhou <aprilzhou@google.com>
Cr-Commit-Position: refs/heads/main@{#1352163}