gui: Set up libgui_mocks

Move the existing mocks for IGraphicBufferProducer and
IGraphicBufferConsumer over to libs/gui/..., and create a new static
library "libgui_mocks" to hold the mocks.

libsurfaceflinger_unittest is also adjusted as needed to use the new
library.

Test: atest libsurfaceflinger_unittest
Change-Id: I740ae9d999812896f525f86a30c6800f86ed8ca9