Separate MemMap from DexFile completely

Create a container class for holding ownership of MemMap and segregate
that functionality to dex_file_loader.  This removes the dependency
between dex_file.cc and mem_map.cc.

Bug: 22322814
Test: make test-art-host
Change-Id: I96db6fd10cdbad774c2f1f85c249418a154fbd52