Fix deadlock between attachSystemDataTransport and addOnTransportsChangedListener.

Thread 1: CompanionTransportManager#attachSystemDataTransport -> (hold mTransports lock) -> notifyOnTransportsChanged -> (wait on mTransportsListeners)
Thread 2: CompanionTransportManager#addListener(IOnTransportsChangedListener listener) -> (hold mTransportsListeners lock) -> getAssociationsWithTransport -> (wait on mTransports)

This change is to use the same lock for both mTransports and
mTransportsListeners.

Test: Can no longer reproduce the deadlock in a test I am writing for wearable sensing.
Bug: 374165899
Flag: EXEMPT bug fix
Change-Id: Ife7fec38d163d81c9039e84406b2d12dcef518c8