Refactor Dex open methods

Consolidate dex opening around a simpler single interface in DexFile.
Making this change prior to moving all but the simplest interface out of
dex_file.{cc,h}.

Bug: 22322814
Change-Id: I4c55019912a19a1c90f9022c20f5b8b954fc7a47
Test: test-art-{host,target}