Performs basic APEX validation in the merged target files package.

Uses apex_utils.GetApexInfoFromTargetFiles to find and parse APEX files
in the target files partition dirs. Raises an error on failure to parse
or duplicate package names.

Bug: 177225446
Test: releasetools_test
Test: Create a merged build that provides the VNDK APEX on both vendor
      and system. Observe failure.
Change-Id: I1356e263b7b32d6063129e079f3ba7ab4ff132a7