Avoid race condition when broadcasting device list changed.

Since broadcastDeviceListChanged could be called in different threads,
there would be race condition causing mutilple callback due to
mPreviousPorts is not thread safe.

Bug: 80138804
Test: run TestDeviceList app in toolbox and cts
Change-Id: I0aa70dc45594bca263ea6f36703f22fe0293f679