[DO NOT MERGE] 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: Ife2441c7322e51ecfb20e0df03dacf6bce49578e