Made the pid number more readable

Changed the name from source to pid, to make it easier to read/understand.

Change-Id: I5ab70dab5694b3bff72ee8b120c35532df34cbdd
diff --git a/aos/events/aos_logging.cc b/aos/events/aos_logging.cc
index f831071..d312925 100644
--- a/aos/events/aos_logging.cc
+++ b/aos/events/aos_logging.cc
@@ -19,7 +19,7 @@
           builder.add_message(message_str);
           builder.add_level(
               static_cast<::aos::logging::Level>(message_data.level));
-          builder.add_source(message_data.source);
+          builder.add_source_pid(message_data.source);
           builder.add_name(name_str);
 
           message.Send(builder.Finish());
diff --git a/aos/logging/log_message.fbs b/aos/logging/log_message.fbs
index 789724f..7e246cf 100644
--- a/aos/logging/log_message.fbs
+++ b/aos/logging/log_message.fbs
@@ -18,7 +18,7 @@
   level:Level (id: 1);
 
   // Pid of the process creating the log message
-  source:int (id:2);
+  source_pid:int (id:2);
 
   // Application name
   name:string (id:3);