SurfaceFlinger: Add "Abstract" Layer type.

Add an Abstract Layer type which exists only to be a parent. We
will use this to more cleanly represent the various hierarchy Layers
from the WM and for an upcoming screenshot rework.

Bug: 72760590
Test: Existing tests pass
Change-Id: Icd19ab0a467585660a95ae37b9c5dd4b8106da9d
5 files changed