tracing protocol: Introduce no_flush

See go/perfetto-frozen

Bug: 308609435
Test: TracingServiceImplTest.NoFlush
Change-Id: I7dfa2e313bf0d478766145c41f79d7dc75d88cd6

CHANGELOG

diff --git a/CHANGELOG b/CHANGELOG
index 777d115..6c75a5c 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,6 +1,9 @@
 Unreleased:
   Tracing service and probes:
     * Added reporting of TZ offset under system_info.timezone_off_mins .
+    * Added no_flush option to DataSourceDescriptor to avoid unnecessary IPC
+      roundtrips to flush data sources like track_event that rely uniquely on
+      server-side scraping.
   Trace Processor:
     *
   UI: