Fix dex file dependencies for oat tests.

The initial prerequisit used as an argument to "cp" wasn't set causing dex2oat
to attempt to compile dex2oat. Make the setting of host and target dex
dependencies more explicit in build-art-test-dex and update the test rules
appropriately.

Change-Id: I4f591f817537043bcb0328c7623fe7e2f0830b60