Device Manager: Connected devices are cleared when the sound indicator is updated

## What Happened

This may just be limited to development, or may extend to normal update installation. When wingpanel-indicator-sound is installed, the device lists are cleared. They can be repopulated by reconnecting the devices or restarting wingpanel.

## Expected Behavior

Installing updates to wingpanel-indicator-sound should maintain its current state of known devices.

## Steps to Reproduce

1. Connect more than one device so the device list shows.
2. Build and install wingpanel-indicator-sound.
3. Device lists are removed.

### Logs

None.

## Platform Information

Compiled from git