Use ConcurrentHashMap to make private maps thread safe

Also synchronize listener un/registering to prevent accumulation of listeners

Fix: 365068571
Test: presubmit crash detector not triggered
Test: TaskVisualsChangedDelegateTest
Flag: com.android.launcher3.enable_refactor_task_thumbnail
Change-Id: Ib15f44d8bca0e24173db3a16bc9184e73e30f4b4