fix a bug where surfaceflinger and system_server could deadlock

    because surfaceflinger handles screenshot in a different
    thread from the binder thread that requested it and because
    the IGraphicBufferProducer is a synchronous interface
    calling back into the system server; it is possible for
    the latter to run out of binder threads (b/c it holds
    a lock while calling into SF).

    The solution is to make sure all calls on IGraphicBufferProducer
    happen on the incoming binder thread. We achieve this by creating
    a IGBP wrapper which is given to the screenshot code.

    Bug: 8734824

Change-Id: I2be85660d9dc65d239d68f6d3ab3c973c13b34cc