Unbreak verifying v2 signatures of large APKs.

The original implementation of APK Signature Scheme v2 verification
mmapped the whole APK. This does not work on devices with limited
amount of contiguous free logical memory, especially on 32-bit
devices where logical address space is relatively small. For example,
a 500 MB APK is unlikely to mmap on a Nexus 6.

This commit fixes the issue by switching the verification strategy
to mmapping each individual 1 MB chunk of the APK, digesting it, and
then immediately munmapping. This is about 5-10% slower than mmapping
the whole APK in one go.

Bug: 27613575
Change-Id: I4167d5a7720c1bb87a0edad5d4f2607f7d554b56