floss: Enable DBus logging for UnregisterScanner

UI used to call UnregisterScanner rather than StopScan. Enabling this
one shall be more helpful.

Bug: 298149797
Tag: #floss
Test: mmm packages/modules/Bluetooth
Test: Toggle FP switch on the UI
Flag: EXEMPT, Floss-only changes
Change-Id: I770a98b1c9267942fe28a7f34d1bd2ea3171586b