ART: Refactor oat_file.h/cc for better maintainability

Refactor the code so that generic oat file initialization (checking
ART symbols etc) is common between dlopen and ART's ElfFile
implementation. Reduce methods and fields exposed in the oat_file
header.

Change-Id: I5bf65dd8b7047a007c6bf435b55bdde306595e8d