[hardening] Use read() instead of mmap() for incfs files

Digest calculation is currently optimized for perfromance, and
it uses fine-tuned mmap() to access the APK. As IncFS-based APKs
aren't supposed to be fully digested, this code will probably
never run for those. But just in case, it's better to switch
over to a slightly slower read()-based implementation that
doesn't have any chance of crashing the whole process via a
SIGBUS signal

Bug: 175632872
Test: atest framework/base/core
Change-Id: Id3dc4cd5090679120d95da40c0dfdde0d72b6bb0