Use SHA-256 for file checksums
Building on !13 and fixing #10670, this adds support for SHA256 file checksums and sets SHA-256 as the default checksum algorithm. Note that SHA-256 (and SHA-512 for that matter) signatures were already supported in APK.
As before, the first supported and trusted signature is used, and the longest supported checksum attached to each file is used. All others are ignored.
This does not necessarily create any APKs with these checksums embedded; that'll take some work over in abuild.