SurfaceFlinger: Add NULL check for buffer handling

Add buffer handling NULL check in dequeueBuffer and
verify whether the output data from binder is not NULL
in queueBuffer and connect api's to avoid SF crash

CRs-Fixed: 573088
CRs-Fixed: 572315

Change-Id: I41cebbc0cbcbbb0fd5ecb38db7ec7b0c91cdffe9
SurfaceFlinger: Enable Vsync trace for SF Composition

Enable traces to show s/w vsync which is used for composition
by surfaceflinger, this helps in identifying the start of
composition cycle in systrace

Change-Id: Id892ca03bacd9f0cfe5ca2c3f8bb5aa892b37231
SurfaceFlinger: Change to support framebuffer flip for 2D blitters

- Surfaceflinger does not flip framebuffers when there are
  no layers marked for HWC_FRAMEBUFFER
- This change checks for the HWC_BLIT flag and will request a flip
  to a new FB_TARGET buffer even if there are no FRAMEBUFFER layers

Change-Id: I1cb44389a05c9ec049d7f0d39c288feccb11a91c