floss: Refactor DisconnectWatcher to take id

Refactor disconnect watcher to associate an id with a callback for
disconnection. This will allow us to verify that a disconnect request
for an id is allowed for a specific client.

Bug: 201599762
Tag: #floss
Test: Verify btclient continues working on ChromeOS
Change-Id: Ia019ad2d9af82be6e1e4eb2d7399bdaadb1600ab