[TabGroupSync] SavedTabGroupKeyedService now owns a proxy service

Renames TabGroupServiceWrapper to TabGroupSyncServiceProxy.

To reduce code complexity we change SavedTabGroupKeyedService so it now
owns a TabGroupSyncServiceProxy which implements TabGroupSyncService.
This new class serves to forward TGSS calls into STGKS calls.

The alternate TabGroupSyncService code path has been removed from the proxy. Only the SavedTabGroupKeyedService should use this path.

This removes the need to return a std::unique_ptr when trying to get the TabGroupSyncService for a profile.

Previous usage: tab_groups::TabGroupServiceWrapper::GetForProfile()
New usage: tab_groups::SavedTabGroupUtils::GetServiceForProfile()

This new function now returns a TabGroupSyncService* instead of a
std::unique_ptr<TabGroupServiceWrapper> which means we do not allocate
memory each time we call GetServiceForProfile() (this function has also
been moved to tab_groups::SavedTabGroupUtils as it is a more centralized
location).

Existing callsites have been updated to reflect these changes (use the pointer instead of the std::unique_ptr).

Other small changes include:
- Updated comments on the classes and member variables to more
accurately describe their functions
- Assignment of raw_ptr member variables to nullptr in header files
- Renaming of variables
- Prefer using the type instead of auto for callers that need to get the service

Change-Id: I175aca42018cdd9bd46ab70480441c81bb70a351
Bug: 359715038
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5787557
Reviewed-by: Emilia Paz <emiliapaz@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Eshwar Stalin <estalin@chromium.org>
Reviewed-by: David Pennington <dpenning@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Darryl James <dljames@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1343077}