JAR signing logic for the apksigner-core library.

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 JAR signing (aka v1 signing) code which
will be used by the future APK signing abstraction exposed by this
library.

All classes 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: I5cecd435e63aab206d63868be5e0d0e289e7c423