Do not try to compile resource-only dex files.

This changes behavior in the case where we are asked to load a dex
file that does not exist or has no classes.dex entry.

Previously we would run dex2oat, which would log an error message and
fail. Now we skip running dex2oat, we report the DexOptStatus as
kNoDexOptNeeded, and we do not try to fall back to the missing
original dex files.

Bug: 21722039
Change-Id: I90b1f4165138daac57bb9d7a354319005652c593