Add missing GUARDED_BY to mdns am: 368704366e am: 31c53b1eb1

Original change: https://android-review.googlesource.com/c/platform/packages/modules/adb/+/3536682

Change-Id: Ie8eeec7bc776f1955d1ceeb716a2a281ff06b0ff
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>