Allow for dependency injection in sharing manager instances.

To support instance-specific sharing behavior and maintain future
flexibility, SharingManagerImpl can be injected with any combination of
underlying managers (i.e. focused tab, focused browser, pinned tab)
during instantiation. Future change to handle wrapping sharing managers
so sharing manager instances can be swapped out on the fly (without
clobbering existing subscriptions).

Bug: 444463509, 445189262
Change-Id: I297b952cad8e496e1f3c80edc90453d08bf07a0f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6952078
Reviewed-by: Erik Chen <erikchen@chromium.org>
Commit-Queue: Marke Hallowell <wry@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1515791}