Destroy DeviceWatcher _after_ stopping discovery.

This lets DeviceWatcher receive InterfacesRemoved signals so that we
can potentially get rid of lost devices.

internal/platform/implementation/linux/ble_v2_medium.cc

@@ -419,16 +419,17 @@ BleV2Medium::StartScanning(const Uuid &service_uuid,
               << "' and message '" << e.getMessage() << "'";
         }
 
-        active_adv_monitors_.erase(service_uuid);
-
         auto &adapter = adapter_.GetBluezAdapterObject();
+        absl::Status status;
         try {
           adapter.StopDiscovery();
+          status = absl::OkStatus();
         } catch (const sdbus::Error &e) {
           DBUS_LOG_METHOD_CALL_ERROR(&adapter, "StopDiscovery", e);
-          return absl::InternalError(e.getMessage());
+          status = absl::InternalError(e.getMessage());
         }
-        return absl::OkStatus();
+        active_adv_monitors_.erase(service_uuid);
+        return status;
       }});
 }
 

internal/platform/implementation/linux/bluetooth_classic_medium.cc

@@ -81,15 +81,16 @@ bool BluetoothClassicMedium::StopDiscovery() {
   auto &adapter = adapter_.GetBluezAdapterObject();
   NEARBY_LOGS(INFO) << __func__ << "Stopping discovery on "
                     << adapter.getObjectPath();
-  device_watcher_ = nullptr;
+  auto ret = true;
   try {
     adapter.StopDiscovery();
   } catch (const sdbus::Error &e) {
     DBUS_LOG_METHOD_CALL_ERROR(&adapter, "StopDiscovery", e);
-    return false;
+    ret = false;
   }
+  device_watcher_ = nullptr;
 
-  return true;
+  return ret;
 }
 
 std::unique_ptr<api::BluetoothSocket> BluetoothClassicMedium::ConnectToService(