Floss: Also stop scan when scanner is unregistered

Add missing behaviors and bug fixing:
* When scanner callback is removed, associated scanners must also be
  unregistered.
* When a scanner is unregistered, that scanner must also be stopped.
* Fixes a bug in btclient's `unregister-scanner` command.
* Removes unnecessary bookkeeping of scanner's UUID since it's already
  kept as the map keys.

Bug: 233124021
Tag: #floss
Test: Manual - Test with btclient

Change-Id: I2fbf14941f6c08214e68db35d97c927de315075f