[SilkFramework] Bug 1048667 - vsync protocol and dispatch framework
1. Add lock in Hwc2D::InitHwcEventCallback(). Call InitHwcEventCallback() in both Hwc2D::init() and GonkVsyncTimer.
2. VDClient will disable the vsync event if it receives MAX_UNUSED_VSYNC_TICK vsync event without any observer.
3. refine comment.