[floss] Properly identify primary services

Primary service is based on the service type and not by being the root
service instance.

BUG=b:270385079
TEST=tools/autotest.py -C out/Default --run_all ./device/bluetooth/floss

Change-Id: I9b3fb01d1e6455e2de2f1f05a0fe174a96878e7f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4304673
Reviewed-by: Sonny Sasaka <sonnysasaka@chromium.org>
Auto-Submit: Abhishek Pandit-Subedi <abhishekpandit@google.com>
Commit-Queue: Abhishek Pandit-Subedi <abhishekpandit@google.com>
Cr-Commit-Position: refs/heads/main@{#1114035}