Floss: filter unexpected cancel discovery from UI

Cancel discovery requests from UI could interrupt the service discovery
process with classic devices. As libbluetooth enqueues start discovery
request when there's ongoing device or service discovery, we use the
discovery state reported by libbluetooth to filter unwanted cancel
discovery requests.

Bug: 232058607
Tag: #floss
Test: pair classic/BLE keyboard and mouse from ChromeOS UI
Change-Id: I345b362ce953f83db6336336b5c26e0d2ae1fd1a