Refactor oat.h.

Move OatClassType and OatMethodOffsets to oat_file.h and
make all dependencies on oat.h explicit instead of relying
on transitive #includes. This reduces the number of files
that need to be rebuilt for oat version changes.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Change-Id: I89c8e397a4f423043d7a036a995cc33548b25051
17 files changed