Update mojo_service_manager BUILD.gn to allow depnding on test_support.

Currently on !use_real_chromeos_services environment,
both mojo_service_manager and fake_mojo_service_manager targets
provides fake_mojo_service_manager.

To avoid it, this CL introduces intermediate targets and unique
the target that provides fake_mojo_service_manager.

This is preparation of ash refactor effort.

Bug: 335294371
Test: Tryjob
Change-Id: I0285e09653028e6ed1e8605e6fbe58357ae0a67d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5712217
Auto-Submit: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Chung-sheng Wu <chungsheng@google.com>
Commit-Queue: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1328619}