SF: Introduce new frontend logic

Changes to allow creating layer snapshots using the new
and legacy frontend logic. Switching the logic is controlled
by debug flags. By default SF will continue to use the
legacy logic so there should be no functional changes with
this cl.

Bug: 238781169
Test: presubmit

Change-Id: I33a4de1b8e93fbfe02ac1787d57d6e07a8c3ef26