Do not prematurely evict segments from multi-period manifests

presentationTimeline.getSegmentAvailabilityStart() yields a start time
relative to the AST, but all of the reference start/end times are relative
to their PTO. We need to normalize the start time to the period before
we evict any segments.

lib/dash/segment_template.js

@@ -80,8 +80,8 @@ shaka.dash.SegmentTemplate.createStream = function(
         context.periodInfo.duration, references);
     if (segmentIndex) {
       segmentIndex.merge(references);
-      segmentIndex.evict(
-          context.presentationTimeline.getSegmentAvailabilityStart());
+      var start = context.presentationTimeline.getSegmentAvailabilityStart();
+      segmentIndex.evict(start - context.periodInfo.start);
     } else {
       context.presentationTimeline.notifySegments(
           context.periodInfo.start, references);