Implement Play Store's "endsley" patching format generator.

Android's Play Store uses, among other options, a bsdiff patch format
similar to Matthew Endsley's bsdiff implementation. The main difference
is that Play Store's version doesn't do any compression in the patch
format itself, but instead the compression is handled at a higher
level.

This CL introduces a new PatchWriterInterface derived class to write to
this (uncompressed) format.

See https://github.com/mendsley/bsdiff for the original implementation
of this format. See also Google's APK patch size estimator for more
information on the file-by-file format used by Play Store:
https://github.com/googlesamples/apk-patch-size-estimator

Bug: 67458097
Test: Added simple unittests for the generated files.

Change-Id: I3b048abf2220e2cb3af0a95e27efe1c0f241046a