APK Signature Scheme v2 signing logic for apksigner-core.

apksigner-code library will offer a high-level primitive (future
commit) for signing APKs. This is meant to be used by
build/tools/signapk and Android Studio's APK builder/signer.

This commit adds a lower-level APK Signature Scheme v2 (aka v2
signing) code which will be used by the future APK signing abstraction
exposed by this library.

All classes (except DataSource and DataSources) added by this commit
are internal (i.e., implementation details of this library). Clients
of this library should not be using these classes.

Bug: 26516150
Change-Id: I98d4da0666cf122667c67565108ea4fb28ac51e6