SF: Allow SurfaceFlinger creation to be altered

As a first step, we need to be able to build a version of the
/system/binsurfacefligner binary that can (eventually) invoke the
SurfaceFlinger constructor to pass a "factory" class to replace certain
key types.

The unittest will also be able to use it to construct mocks by default.

Bug: None
Test: marlin builds, cheets_x86_64 builds (and overrides default)

Change-Id: Id6d5cabf0dd54e85ac0e6706edf3cbe8a0288bc3