[layertracegenerator] be more resilient to unknown layer ids

While unexpected, the layer lifecycle in legacy which is used to
generate the transaction traces is less explicit. We rely on layer
destruction when the object becomes unreachable. The new
front end logic will crash if called with an unknown layer id.
While we have to interop between both sets of logic, log and
ignore the unknown layer ids so we can still generate useful
layer traces.

Fixes: 275630566
Test: presbumit
Change-Id: I5b2a217c78b4a9e94f0772459d6aa0c68a8a51d0