[silk] enable silk and vsync-aligned RD and compositor pref
Bug 1092978 - Part1: [Silk] get vsync rate from HwcComposer2D
Bug 1092978 - Part2: [Silk] add registry interface for vsync-aligned refresh driver.
1. create "gfx.vsync" pref to turn/off silk.
2. query vsync rate from hwc.
3. provide registry interface for RD.
4. do not hold the observer's ref-count.
5. change VsyncDispatcher init flow.
Bug 1092978 - Part3:[Silk] add a PVsyncEvent sub-protocol in PBackground for cross process vsync event passing.
Bug 1092978 - Part4: [Silk] vsync-aligned refresh driver

1. add "gfx.vsync.refresh-driver" pref to turn on/off vsync-aligned refresh driver
2. observe vsync event to tick refresh driver.
[Silk] wip, PlatformVsyncTimer
add systrace tracer
add systrace tag for RD and vsync event
debug DidComposite() ipc message
debug CompositorParent composing time