Create an ART-independent DexFileLoader

Opening DEX files should not rely on instantiating a runtime or having a
large number of dependencies on runtime components.  This CL makes
DexFileLoader a stub class that is independent of ART, and introduces a
subclass ArtDexFileLoader that contains the current implementations.

Bug: 22322814
Test: make -j 50 test-art-host
Change-Id: Ia6e92ae93c347057ea0c10455525239cbbe42c03