VolumeDialogImpl: Handle cases where media controller is not available

As of QPR2, we can't crate a media output dialog with empty package
name unless the use_media_router2_for_info_media_manager flag is
enabled.

When the flag is enabled, we can always show the dialog by passing
the actual package name, or null if the media controller is not
available.

If the flag is disabled, we can only show the dialog if the media
controller is available.

Change-Id: Ic8d08905bf55be91b547708b316ef7c96c81c492
Signed-off-by: danielml <daniel@danielml.dev>