bluetooth/gst: Timestamp encoding buffers according to PA clock

Commit c6d6ca54 ("bluetooth/gst: Replace buffer accumulation in adapter
with direct pull") removed the `timestamp` parameter from GStreamer
transcoders due to being unused, but these should instead be propagated
to the GStreamer encoding buffers.

Part-of: <pulseaudio/pulseaudio!494>
2 jobs for gst-synchronous in 3 minutes and 34 seconds (queued for 48 seconds)