SF: Add and use MockGraphicBufferProducer/Consumer

This has the benefit of allowing the tests to set expectations on the
producer/consumer queue calls.

Additionally this speeds up test execution (though possibly only for the
first test which creates a buffer queue) by a factor of 4x.

[Test execution time reduced from 25ms to 6.1ms]

Test: libsurfaceflinger_unittest passes on Pixel XL
Bug: None
Change-Id: Ic400e8d123fea497061c193df5036218ebef1d3a