[tools] Add a tool to decompress XA-compressed assemblies

[grendello]

The utility is named decompress-assemblies and takes as its
parameters a list of files to decompress. The files might be either
.apk archives or .dll files.

I case of .apk, managed assemblies stored in the assemblies/
directory inside the APK will be read and decompressed.

The uncompressed result is stored in the current directory with the
original assembly name modified to contain the uncompressed- prefix,
in order to prevent accidental overwriting of assemblies found in the
current directory.

[jonathanpeppers]

I don't think you need to set $(LangVersion) with $(LangVersion).

This imports Configuration.props, so I think it would get the value you want;

https://github.com/xamarin/xamarin-android/blob/1877dd5ebee90452c6ab2cc33a58498a83ea7c1f/Configuration.props#L38

[radekdoulik]

This can be handy to extend to handle .aab files as well.

[grendello]

Support for AAB added :)

[jonpryor]

Should we add this tool to the installer?

[jpobst]

I would suggest the opposite: remove it from xamarin-android, put it in a separate repo, and make it available as a dotnet tool like we do for apk-diff. (https://github.com/radekdoulik/apkdiff)

I think the last thing we want to do it add another one-off tool to our maintenance burden.

[jonpryor]

I think the last thing we want to do it add another one-off tool to our maintenance burden.

There's two related considerations, which also apply to PR #5541:

1. These utilities are as much for our own use as for anyone else. We need them for diagnostic purposes.
2. Where the utilities are located doesn't change the fact that we need to maintain them, as per (1).

I think the more important question is one of "tying": if "something internal" changes in this repo, will the tool need to be updated?

In the case of decompress-assemblies or tmt (PR #5541), the answer is "Yes!". If the compression format changes, the the tool needs an update. If the typemap format changes, then the tool needs an update.

In both cases, having the utility in the same tree makes it easier to review the changes and ensure that the tool continues to work.

Compare to apkdiff, which doesn't rely as much on xamarin-android internals.