Pass the right dex files to dex preopt.

We currently pass the boot dex files installed on /system, but these
may be eg stripped. That's currently OK as dex2oat is looking
at the dex files in boot.oat, but dex2oat could look at them for
various reasons.

Test: m
Change-Id: I85915c979cf94ec29751b10a0d8a5957b6cea956