Fix stream metadata update via MetaProxy.

Do to thread affinity EngineController::currentMetadataChanged() signal
was never delivered to MetaStream::Track::Private.
Solved by forcing a direct slot connection.

Note: Stream metadata changes while playing is broken in Phonon-gstreamer
4.6.0 but it's fixed in 4.6.1 according to tdfisher.

BUG: 300334