Simplify decision of which is the best oat file.

Previously the best oat file was determined by looking at all
combination of states of the oat file in the odex location and the oat
location. This change simplifies the decision by using the oat
location if that oat file is not out of date and using the odex
location otherwise. For practical cases, both decisions lead to the
same result.

Bug: 30937355
Test: oat_file_assistant_test

Change-Id: I687206612914cdbd43950fcdd7dd818a54255a6b