sde: Add support for DisplayEventHandler

- Add DisplayEventHandler to receive VSync event from driver.
- Add hooks to enable/disable VSync from client.
- Propogate VSync timestamp to the client

Change-Id: I27ea9773c424d8ad85f11f292ddd8792564bbcdc

displayengine/include/utils/constants.h

@@ -45,6 +45,8 @@
 
 namespace sde {
 
+  const int kThreadPriorityUrgent = -9;
+
   typedef void * Handle;
 
 }  // namespace sde