[apksigcopier] support gradle/zipflinger signing as well?

Some apps are not signed by apksigner, but (presumably) by gradle.

This results in ZIP (meta)data differences wrt apksigner output that apksigcopier currently does not handle.

There is some work in progress upstream for handling this, which works for these 2 specific apps, but there are some unresolved issues:

• I currently only know of 2 examples of APKs with a (132-byte) "virtual entry" at the beginning of the signed APK.
  • why does zipflinger add it (i.e. why is there "empty space" for it to fill)? why is is 132 bytes?
  • could the "empty space" have been added by some other tool before signing? it's not in the f-droid unsigned APKs.
• I'm therefore currently unsure whether supporting this even makes sense.
• I'd certainly need like more examples to test against.
• I'd like to know exactly how these APKs were signed.
  • AFAIK we don't know with 100% certainty it was gradle and not some other tool.
  • and what exactly does gradle use to sign these APKs?
• I currently don't have an android dev environment in which to test signing with gradle, so could use some help reproducing this 😅