Allow multiple listeners for the same message type

We need this to support multiple concurrent CDM Secure Channels in
WearableSensingManagerPerUserService. They all listen for messages of
type MESSAGE_ONEWAY_FROM_WEARABLE

Test: A new test I am writing for concurrent wearable sensing connection
passes after this change.
Flag: EXEMPT small change
Bug: 373945898

Change-Id: I2aad20f4aec2c76e50d864d146a259f6363ee1dd