Put boot class loader classes and strings in dex cache of app images.

The reason for b/28295348 was that an ArtMethod of the boot image was
in the app image dex cache, but the declaring class of that
boot image method was not.

Since objects of boot images don't need fixups, the comparisons for
FixupStrings and FixupResolvedTypes was always false for them.

bug:28295348

Change-Id: Ida6d42c902692914c302b7fe3d003f73710de6e1