rework screenshot API and implementation

- SurfaceFlinger now supports to take a screenshot
  directly into an IGraphicBufferProducer

- reimplement the IMemoryHeap screenshot on top
  of the above

- reimplement LayerScreenshot such that its
  BufferQueue is directly used as the destination
  of the screenshot. LayerScreenshot is now a thin
  wrapper around Layer

Bug: 6940974

Change-Id: I69a2096b44b91acbb99eba16f83a9c78d94e0d10