Add log for unregister scanner success

Currently there are no callbacks for when unregistering scanner is successful. This makes debugging hard when multiple scans are ongoing at the same time, since we can't distinguish which scanner was unregistered. Thus adding a log in the success case.

Bug: 346886818
Test: m com.android.btservices
Flag: EXEMPT, adding logs only
Change-Id: I43dcb529f60eca9c032d0070a1e59ae8d1ff50ed