Change to SynchronizedList to fix exception.

In systemui, it use multithread to access LocalBluetoothManager,
so we need to use synchronizedList to remove
ConcurrentModificationException.

In b/116662744, we will refactor multi-thread synchronization once
hearing aid refactor is done.

Bug: 117122392
Test: Manual
Change-Id: I50cc09c2ed9631072ed6aff13585fda116dc02de