[floss] FlossManagerClient: Don't re-register callbacks on ObjectAdded

We re-register the callbacks on ObjectAdded because there was an issue
that ObjectRemoved wouldn't be called, which has been fixed in
b:193839304.

BUG=b:308362469
TEST=Restart UI, verify the callbacks are only registered once
TEST=Restart btmanagerd, verify UI works fine

Change-Id: Ie2e31e1882f3e554257eebad9dcf2e5eb4a890da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4988751
Reviewed-by: Katherine Lai <laikatherine@chromium.org>
Commit-Queue: Hsin-chen Chuang <chharry@google.com>
Cr-Commit-Position: refs/heads/main@{#1217440}