Touch up changes related to cross-dex compiling * Added exaplanatory comments * Used IsBootStrapClassLoaded to avoid a read barrier * Use the new multi-image from CompilerOptions Bug: 154012332 Test: art/test/testrunner/testrunner.py --host --64 --optimizing -b Change-Id: I10a4ed0f9d5b2e796102846564a54dc90f297068