Remove #include "oat_file.h" from class_linker.h .

Refactor the OatClass and OatMethod related functions from
ClassLinker to OatFile and ArtMethod, respectively. Refactor
the remaining ClassLinker dependencies on OatFile to break
the #include dependency and reduce incremental build times.

Test: m test-art-host
Change-Id: Iebc5b9f81b48fbcf79821cc827a5d7c4a0261bf6