Concurrent collections for Bluetooth callbacks.

This allows callback classes to remove themselves or add additional
callbacks in response to state change dispatches.

Bug: 129060225

Test: build and deploy, pair multiple devices (w/code path that hits
 problem case), RunSettingsLibRoboTests

Change-Id: I8eed81bbc9c12321ec41b2491d006764e2e483d6
2 files changed