Add a shared interface for different types of dex files

Repurposed DexFile to be the shared API. Will be used for abstracting
accesses to original dex files and CompactDex files.

As implementation diverges, functionality will be moved from DexFile
accordingly.

Will consider renaming DexFile -> IDexFile in a follow up CL.

Bug: 63756964
Test: test-art-host

Change-Id: Iad2508c2b9a7b6e0669fca5f7d10299a9b1541a2