playbin{2,3}: fix base_time selection when flush seeking live

This is a direct translation of
as playbin{2,3} insulates its sub groups state changes from the pipeline
base class, it needs to track whether the subgroup is live itself,
and handle RESET_TIME the same way GstPipeline does.

Part-of: <gstreamer/gst-plugins-base!1110>
33 jobs for !1110 with backport-mr-1104-into-1.18-playbin-reset-time in 43 minutes and 27 seconds (queued for 15 seconds)
merge request