SF: Add a first real unit test

Use the newly defined mocks to substitute for the real instances,
allowing a simple direct test of
SurfaceFlinger::processDisplayChangesLocked(), for the case of a display
being added.

[Test execution time 25ms]

Test: libsurfaceflinger_unittest passes on Pixel XL
Bug: None
Change-Id: Icdb763ecc3adb1a8bf04268f3dc5716606e6e7e8