Prevent crashes if boot images fail to load

We now reset the requested_alloc_space_begin so that there are no gaps
between any boot images and the non moving space.

Also clear boot_image_spaces so that ClassLinker::InitFromBootImage is
not called.

Finally, delete image spaces so that the maps and oat files are
unmapped.

Bug: 26913820

Change-Id: If5058a214456bd123163ae064a0c66d6a77d6309