ART: Check for more low-space situations in zygote

Even if a boot image exists and can be loaded, the rest of the
platform may not be able to boot. We previously only checked
after full image generation and missed this case.

Also prune the cache if there wasn't even space to put the boot
marker.

Bug: 30765660
Change-Id: Icd43746e681c62ce2e4e8745fb17bda65ca60372