SF: Remove BufferStateLayer class

This CL removes the BufferStateLayer class and some of the references to
"BufferStateLayer" in factory and utility method names. In order to keep
the diff smaller, I'll send out a separate CL to remove the remaining
references.

Bug: 238781169
Test: refactor, existing tests pass

Change-Id: I62fe00caa87e109435da62329e817c19cfb96ead