utils: clarify scoped tracing functionality

ScopedTrace objects were being used in place of ATRACE_NAME because of a
misunderstanding of it's function.  Cleared up documentation for usage.

Also explicitly use global namespace for sysprop callback.

Change-Id: I7c248b486b614ccdb841659ca0dcfc644fda670a
diff --git a/libs/utils/Trace.cpp b/libs/utils/Trace.cpp
index d4f0218..36fd802 100644
--- a/libs/utils/Trace.cpp
+++ b/libs/utils/Trace.cpp
@@ -21,5 +21,5 @@
 
 static void traceInit()
 {
-    android::add_sysprop_change_callback(atrace_update_tags, 0);
+    ::android::add_sysprop_change_callback(atrace_update_tags, 0);
 }