fix an issue with vsync event delivery

vsync events were sometimes delivered to connected
client who didn't request them. this happened if
another client requested the delivery and that client
was first in the client list.

also fix the vsync test which didn't request any events as
well as DisplayEventReveiver documentation which was misleading
about the necessity to request vsync events.

Change-Id: Ie990fda3f337f8f0042745c4b2cde67936c45686
diff --git a/services/surfaceflinger/EventThread.cpp b/services/surfaceflinger/EventThread.cpp
index 92d4266..af0da0b 100644
--- a/services/surfaceflinger/EventThread.cpp
+++ b/services/surfaceflinger/EventThread.cpp
@@ -151,9 +151,9 @@
             mLastVSyncTimestamp = timestamp;
 
             // now see if we still need to report this VSYNC event
-            bool reportVsync = false;
-            size_t count = mDisplayEventConnections.size();
+            const size_t count = mDisplayEventConnections.size();
             for (size_t i=0 ; i<count ; i++) {
+                bool reportVsync = false;
                 const ConnectionInfo& info(
                         mDisplayEventConnections.valueAt(i));
                 if (info.count >= 1) {
@@ -174,11 +174,7 @@
                     displayEventConnections.add(mDisplayEventConnections.keyAt(i));
                 }
             }
-
-            if (reportVsync) {
-                break;
-            }
-        } while (true);
+        } while (!displayEventConnections.size());
 
         // dispatch vsync events to listeners...
         vsync.header.type = DisplayEventReceiver::DISPLAY_EVENT_VSYNC;