avformat/dashenc: setting @availabilityStartTime when the first frame is ready

@availabilityStartTime specifies the anchor for the computation of the earliest
availability time (in UTC) for any Segment in the Media Presentation.

As per this requirement, the @AvailabilityStartTime should be set to the
wallclock time at which the first frame of the first segment begins encoding.
But, it was getting set only when the first segment was completely ready. Making
the required correction in this patch. This correction is mainly needed to reduce
the latency in live streaming use cases.

libavformat/dashenc.c

@@ -743,9 +743,6 @@ static int write_manifest(AVFormatContext *s, int final)
             update_period = 500;
         avio_printf(out, "\tminimumUpdatePeriod=\"PT%"PRId64"S\"\n", update_period);
         avio_printf(out, "\tsuggestedPresentationDelay=\"PT%"PRId64"S\"\n", c->last_duration / AV_TIME_BASE);
-        if (!c->availability_start_time[0] && s->nb_streams > 0 && c->streams[0].nb_segments > 0) {
-            format_date_now(c->availability_start_time, sizeof(c->availability_start_time));
-        }
         if (c->availability_start_time[0])
             avio_printf(out, "\tavailabilityStartTime=\"%s\"\n", c->availability_start_time);
         format_date_now(now_str, sizeof(now_str));
@@ -1292,6 +1289,10 @@ static int dash_write_packet(AVFormatContext *s, AVPacket *pkt)
     if (os->first_pts == AV_NOPTS_VALUE)
         os->first_pts = pkt->pts;
 
+    if (!c->availability_start_time[0])
+        format_date_now(c->availability_start_time,
+                        sizeof(c->availability_start_time));
+
     if (c->use_template && !c->use_timeline) {
         elapsed_duration = pkt->pts - os->first_pts;
         seg_end_duration = (int64_t) os->segment_index * c->seg_duration;