[sf] provide a unique id for snapshots

Input and layer tracing relies on the fact that all layers
have a unique id. Because mirror layers are no longer
implemented by duplicating layers, we can have multiple
snapshots with the same id. To fix this, provide a unique
id that can be used by consumers who require them.

Test: atest FlickerTest
Bug: 238781169
Change-Id: Ibe54154b6e14f72bfe73a15faa0e7e0d19293e36