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.
2 jobs for gst-synchronous in 1 minute and 49 seconds (queued for 4 seconds)
latest
Status Name Job ID Coverage
  Container
passed build-container #16149157

00:00:10

 
  Build
passed build-meson #16149158

00:01:39