[sf] Fix layer ids for background color layers

Fixes a regression caused by Ie0f93ca956e6d043c9d95d00bc205d242e47c4cc
which created duplicate layer ids.

Also deflakes corner radius tests by tracking eCornerRadiusChanged state
changes correctly.

Test: presubmit w/new fe
Bug: 238781169

Change-Id: I2d5321b5ba77e1074ef039bcd90c78e82c1a0049