Refine OatStatus enum.

Replace Exists() with a separate OatStatus code, and distinguish
between the oat file being out of date due to the dex being out of
date or the image being out of date.

Bug: 30937355
Test: oat_file_assistant_test
Change-Id: Ia3853e461beb07f0b417276b9277b9c562b20865
5 files changed