SF: Set up librenderengine_mocks

Move the existing mocks out of the unit test directory into
RenderEngine/include/renderengine/mock and RenderEngine/mock.

This allows other unit tests to easily reuse the mocks.

Change the unit test to use the new library and reference the moved
headers.

Test: atest libsurfaceflinger_unittest libcompositionengine_test
Bug: 121291683
Change-Id: I1577995e6140cb3949207c5bc746c46b7828ae19