Concurrent collections for Bluetooth callbacks.

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

Bug: 129060225
Bug: 144357642

Test: build and deploy, pair multiple devices, switch users

Change-Id: I8eed81bbc9c12321ec41b2491d006764e2e483d6
Merged-In: I8eed81bbc9c12321ec41b2491d006764e2e483d6
2 files changed