[LayerTraceGenerator] fix mirrors and rel-z data

relanding: actual regression was fixed here: Id58a41b46b4d03dc6d69259220c04e2f341be4fd and was not caused by this cl

Now that we are using the new front end to
generate layers trace, we need to make a few fixes
so we can generate data that's parsable by winscope.

This means use a unique id instead of the layer id to
identify layers.

Test: presubmit
Fixes: 255901752
Change-Id: Ife768ada07940f298a0a7b1ff81d95901559c57c