Remove gaps between images

We now use the same logic to calculate the size as we use to create
the image header. This should mean that it always lines up correctly.
Previously the alignment rounding could have caused page off by one
errors in some cases.

Re-enable no gap check in heap.cc.

Bug: 26317072
Change-Id: I558a4e60197ca4613fc4ec4c3a618c64a83d006a