hwcomposer : Use sysfs to read vysnc information

* Read sysfs entries to get vsync timestamp from kernel.
* External display continues to use uevents for hotplug events
* A new thread, vsyncThread is created to read and send vsync
  timestamps to HAL.
* Disable H/W VSYNC for MDP 5.x targets until kernel changes are
  complete.
* Synchronization is needed to make sure read() is not called
by hwcVsyncThread when VSYNC is disabled.

Change-Id: Iab0a94a3cfce9142b8867859f2a7d9bcaecb0996
Acked-by:  Neti Ravi Kumar <ravineti@codeaurora.org>
diff --git a/libhwcomposer/Android.mk b/libhwcomposer/Android.mk
index 8697b5a..25946ff 100644
--- a/libhwcomposer/Android.mk
+++ b/libhwcomposer/Android.mk
@@ -15,6 +15,7 @@
                                  hwc_video.cpp    \
                                  hwc_utils.cpp    \
                                  hwc_uevents.cpp  \
+                                 hwc_vsync.cpp    \
                                  hwc_mdpcomp.cpp  \
                                  hwc_uimirror.cpp \