Revert^2 "Refactor image loading."

This reverts commit 701b6eec.

And fixes the boot class path retrieval. Monolithic boot
image does not contain the boot class path.

Change-Id: I6978e0f1fa0acda8acd43be007ac1a90a8fd9f98
Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Test: Pixel 2 XL boots.
Test: m test-art-target-gtest
Test: testrunner.py --target --optimizing
Bug: 112827048