Fix verifier deps determinism

Previously, the unverified_classes were not deterministic since
they were represented as a vector. This CL moves it to a set and adds
a regression test.

Bug: 63467744
Test: test-art-host

(cherry picked from commit bf755fef)

Change-Id: I4dce9f95fc1e56e383e8c617e1703ce360c67d5b