Bug 987527 - Register Vsync monitor to HWComposer

In order to enable/disable the Vsync from mHwc, we should make sure
the order of each command is correct. In this patch, I added an
interface for registering these mHwc callback functions and some
accessor/helper functions for manipulating Hwc Vsync.
Merge branch 'silk-hwc' into silk-Bug-1048667

Conflicts:
	gfx/thebes/gfxPrefs.h
	widget/gonk/HwcComposer2D.cpp
	widget/gonk/HwcComposer2D.h