VNDK listing contains device modules only

Fix a bug where host-only modules were incorrectly listed as VNDK.
Also refactor VndkMutator() / apexVndkDepsMutator() module skipping
logic.

Bug: 158543482
Test: Add unit test to cc/cc_test.go
Change-Id: I50b09f526cbc081149d8241c2a091e3ee48ef4d7