surfaceflinger: Trace HW vsync enable

This makes it possible to tell whether two HW vsync signals are too
far apart because one was late, or because we turned HW vsync off
briefly between them.

Bug: 17259382
Change-Id: If4fba2a8a6013568349949ce87c5c36f0468d2a2

services/surfaceflinger/DisplayHardware/HWComposer.cpp

@@ -524,6 +524,10 @@ void HWComposer::eventControl(int disp, int event, int enabled) {
             if (!err) {
                 int32_t& events(mDisplayData[disp].events);
                 events = (events & ~eventBit) | newValue;
+
+                char tag[16];
+                snprintf(tag, sizeof(tag), "HW_VSYNC_ON_%1u", disp);
+                ATRACE_INT(tag, enabled);
             }
         }
         // error here should not happen -- not sure what we should