Settings: Fix assertThat in tests

Truth.assertThat is a fluent-style API. Fix the tests to actually
do something.

Found by errorprone.

(cherry picked from commit 22bad556)

Bug: 73513670
Test: m RunSettingsRoboTests
Test: m javac-check RUN_ERROR_PRONE=true
Exempt-From-Owner-Approval: Cherrypick
Merged-In: Ib923de28ce5dcf79b9dec718306877e31e58e024
Change-Id: Ib923de28ce5dcf79b9dec718306877e31e58e024