ART: Do not relocate app program headers in patchoat.

Change the check whether to relocate program headers in
patchoat to simply look whether there is a PT_LOAD section
with p_vaddr == 0. If there is, don't relocate the headers,
it should be an app. Otherwise, it's a boot image and needs
to be relocated.

Add overflow checking to ElfFileImpl<>::GetLoadedSize().

Bug: 21047854
Change-Id: Ie6737f338687296b6dbf4bb3b36358501dfb1587