[display][coordinator] Fix test flakinesses.

The initial version of the fixes was obtained by asking gemini-cli to
run the following command and fix bugs until the tests pass.

fx test -q display-coordinator-unittests.cm \
    --test-filter=IntegrationTest.ClampRgb --count 100 --parallel 20

Bug: 487682820
Test: fx test display-coordinator-unittests.cm --count 100 --parallel 20
Change-Id: I4166cba02c594f72081f5afd26af55bbe601d70a
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1538600
Commit-Queue: Victor Costan <costan@google.com>
Reviewed-by: Yilong Li <liyl@google.com>
Fuchsia-Auto-Submit: Victor Costan <costan@google.com>