surfaceflinger: Create SurfaceFlingerBE class

Split SurfaceFlinger object into a backend and
frontend version.  This change creates the backend
struct and only puts a single data item in that
struct.  Follow-on changes to move more things
to the backend will follow

Test: build
Change-Id: I0fd6b4eb506791346da2b9f94cc12d9cf854ad08