[sf] fix overflow in snapshot generator

Fixes a couple of issues that caused the snapshot
builder to crash from a stack overflow:
- mirror layers were temporarily rooted to the display
with the default layer stack
- there was no check to prevent a layer from mirroring
itself (causing a cycle in the hierarchy)

Also modifies the layer trace generator to easily generate
test data for the transaction trace test suite.

Test: atest transactiontrace_testsuite
Fixes: 282110579
Change-Id: I2954f838e333b857b4b755e11666c2580d97c2b6