Create OatFileAssistant class for assisting with oat files.

The oat file assistant is used for determining whether dex2oat or
patchoat is needed, for running dex2oat or patchoat as needed to make
an oat file up to date, and to load dex files associated with a given
dex location.

The introduction of the OatFileAssistant class is meant to clean up and
consolidate code related to the management of oat files that was
duplicated and spread across dalvik_system_DexFile.cc and
class_linker.cc.

Bug: 11301553
Change-Id: I0c16027b9bae4570c2c50faa9c14f581c0cbafb8