Remove fuzzer build-time dependency on core image.

Test if the image exists at runtime.

Forcing this dependency slows down development. For example, if after a
change dex2oat does not compile anymore you need to first revert to a
stable state and rebuild the core image to be able to move foreward.
Since this is a test tool, it is ok to assume that the core image has
already been built if host execution was selected.

Change-Id: Id61a960ba62bb8d1d21acb07114f72208fbc9720