Surfaceflinger: Create LayerBE data class

Split Layer class into a backend and front-end 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: I01d217468ea6008eb46cc3a225d47e1e0b90865a
3 files changed