SettingsLib: Fix assertThat in test

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

Found by errorprone.

Bug: 73513670
Test: atest WifiTrackerTest
Test: m javac-check RUN_ERROR_PRONE=true
Change-Id: Ic16cded73ad5192d6c32033e41e72e39277729c6