Propagate the dex jar path as an OptionalPath which is either valid or

invalid with a message.

This will allow propagating any error from the deapexer module for
prebuilt APEXes to the location where the dex jars get used. It's only
at those points that we can raise errors about not being able to
extract files from the deapexer modules if they are invalid, and this
way we avoid encoding knowledge there about why they may be invalid.

To keep the refactoring limited it intentionally does not change any of
the existing logic for when dexJarFiles are set or not (non-nil vs nil
prior to this change), although there may be opportunity to use this
for more conditions when dex jars aren't available.

The refactoring is also not extended to
dexpreopt.ClassLoaderContextMap.

Test: m nothing
Bug: 192006406
Change-Id: I68986dccd9a9b3fee4d24caa1947ea17a36caedc