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

(cherry picked from commit 1e5ff87e)

Change-Id: Ia31937f29ef217eac083165dfff37cd068fbd32b