tracing: Implement system consumer backend

This patch add a consumer backend for the client API, making it possible
to record traces from a system tracing daemon. Note that on some systems
only privileged apps will have read access to the system tracing
service. For example on Android you should use the "perfetto"
command line tool to record system-wide traces instead of the client API.

Bug: 170126760
Change-Id: Ifb4ada4a13290030fb0f3029858fb4c6247c1c62
diff --git a/CHANGELOG b/CHANGELOG
index e37f6ab..e85c3d5 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,6 +1,10 @@
 Unreleased:
   Tracing service and probes:
-    *
+    * Added support for recording traces from a system service through the
+      client API.
+    * The client library now reconnects producers automatically if the
+      connection to the tracing service is lost. Also fixed crashes in ongoing
+      tracing sessions when a disconnect occurs.
   Trace Processor:
     *
   UI: