Update to latest trace-viewer (a7f1c1b3)

Includes rewrite of prefix.html to switch from TraceViewer
to timeline-view, and account for new location of the
vulcanizer.

Cherry-pick of ced05db70069f9d84c4b0dd9b3b26b94e3482336 from AOSP

Change-Id: I07285c64bde7d4bfe414c9e93b097435e118a069
diff --git a/update.py b/update.py
index 8a66066..770fa6c 100755
--- a/update.py
+++ b/update.py
@@ -54,7 +54,7 @@
 build_dir = os.path.join(trace_viewer_dir)
 sys.path.append(build_dir)
 
-from trace_viewer.build import vulcanize_trace_viewer
+from tracing.build import vulcanize_trace_viewer
 with codecs.open(output_html_file, encoding='utf-8', mode='w') as f:
   vulcanize_trace_viewer.WriteTraceViewer(
       f,