ApkSignerEngine abstraction for apksigner-core library.

This adds the ApkSignerEngine abstraction to the apksigner-core
library. This abstraction enables the separation of
parsing/building/writing of an APK from signing the APK.

Users of ApkSignerEngine invoke it during specific I/O events (e.g.,
JAR entry encountered in input APK, all output JAR entries ready).
The engine responds by occasionally providing instructions about what
to do next. For example, it may provide JAR entries to be added to
the output APK, thus making the output APK signed.

The default implementation of ApkSignerEngine will be provided in  a
follow-up commit.

Bug: 26516150
Bug: 27461702
Change-Id: Ia85fd09497a6e2ff3103ba10c53a1848ba731f7b