Replace kotlin 'assert' with Truth 'assertThat'

'assert' may not throw an AssertionError, depending on what flags are
enabled on the JVM.

This introduced misleading assertions and the possibility of failing
tests to display as passing.

Bug: 285532044

Test: atest ResolutionMechanismTest

Flag: TEST_ONLY

Change-Id: I0c58fd043d2d451049a93ea20f89525ae162e651