hwc: support vsync notification via sysfs

Also includes:
    hwc: fix timing issue with vsync sysfs loop

    Change-Id: I6e0307e0fd5ea84748ddb580062b21f697512603

Change-Id: Ie2172b5fb2e8595136f6ccb28abd9f8ce89a526f
Merge "hwc: support vsync notification via sysfs" into android-4.3