[LayerTraceGenerator] Ignore unknown handles

Ignore unknown handles when iteroping with legacy front end. In the
old world, we would create child layers which are not necessary with
the new front end. This means we will get notified for handle changes
that dont exist in the new front end.

Test: presubmit
Bug: 255901752
Change-Id: I669bcd7d635db6fa86cd3a64fc260178c2a85580