Keep dex location from OatDexFile.

This is needed for loading boot image with -Xbootclasspath:
and -Xbootclasspath-locations: for app preopt. These options
are necessary for loading additional components of the boot
class path for preopt with a partial boot image.

Test: Pixel 2 XL boots.
Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Test: m test-art-target-gtest
Test: testrunner.py --target --optimizing
Bug: 119868597
Change-Id: I5e42f85c08eb9126c1045258deacc55fc7eebe00