ART: Fix preloaded dex cache expectations

In fallback mode, there may not be dex caches for all boot classpath
dex files - there is no guarantee that a class from every dex file
has been loaded.

Bug: 36033084
Test: m
Test: m test-art-host
Test: manual - fill the /data partition, reboot
Change-Id: I3752b48d9d61947959e8056bd01db0e323724d37