Update ART runtime to recognize artifacts on /system for APEX.

Context: go/location-for-compilation-artifacts-of-apex-jars

This change enables ART runtime to use the artifacts generated by
aosp/1821984.

Bug: 194150908
Test: atest art_standalone_libartbase_tests
Test: manual - 1. Patch aosp/1828115.
  2. Build a system image and flash it into a device.
  3. adb root && adb shell grep "/system/framework/oat" /proc/`adb shell pidof system_server`/maps
  4. See "/system/framework/oat/x86_64/apex@..." in the output.
Change-Id: I74cb21433d63b1f3ed4d096ab0c86f09bced4b97