Change name of system property for traces.

This allows the property to be changed from the settings UI.

Change-Id: Ife7424b3549e5bbe51b6ad2fb8e5edde3a9fd608
diff --git a/libs/utils/Trace.cpp b/libs/utils/Trace.cpp
index f7d8abe..f3ec485 100644
--- a/libs/utils/Trace.cpp
+++ b/libs/utils/Trace.cpp
@@ -39,7 +39,7 @@
             // sEnabledTags remains zero indicating that no tracing can occur
         } else {
             char value[PROPERTY_VALUE_MAX];
-            property_get("atrace.tags.enableflags", value, "0");
+            property_get("debug.atrace.tags.enableflags", value, "0");
             sEnabledTags = (strtoll(value, NULL, 0) & ATRACE_TAG_VALID_MASK)
                     | ATRACE_TAG_ALWAYS;
         }