Properly support AGP 4.1

[benjamin-bader]

AGP 4.1 introduces a large set of new APIs of interest to us, providing standard (and supported!) ways to get access to a large number of previously-internal intermediate artifacts like built APKs, bundle APKs, obfuscation mapping files, etc.

Especially to unblock deobfuscation, we'll need to use these APIs

Example: https://github.com/android/gradle-recipes/blob/master/BuildSrc/getApksTest/buildSrc/src/main/kotlin/ExamplePlugin.kt

[benjamin-bader]

Well, this is a can of worms. I think we'll need to go to 2.0.0 for this; it's not really possible to emulate the exact output format of the current task with the new AGP version, and the old format is a little busted anyways.

It would be much better to make a clean break and re-think some aspects of the whole plugin.