Fix: Only wire upload mapping task if minifyEnabled

[cerisier]

📜 Description

Fixes #82.

We only wire the upload mapping task if variant buildType has
minifyEnabled to true, which basically hints that proguard
rules will run.

Note that I got rid of variant.register since I believe we do not need it as we already wire the task manually. Happy to leave it there if needed.
Note that we still create the task eagerly for now, which is to be tackled in another PR.

💡 Motivation and Context

This follows a discussion with @marandaneto about #82

💚 How did you test it?

Published locally and ensured green build in debug variant where minifyEnabled is false.

📝 Checklist

• I reviewed the submitted code
• I added tests to verify the changes
• I updated the docs if needed
• No breaking changes

🔮 Next steps

[cortinico]

Should we try to merge #85 instead of this one, wdyt @cerisier?

[cerisier]

@cortinico Sorry I had quickl discussed with @marandaneto this afternoon and was suggesting I'd do a simple PR to fix the most urgent issue and take a bit more time to fix the eager task creation since this is going to be more complex given how things work atm.
Happy with both :)

[marandaneto]

@cortinico Sorry I had quickl discussed with @marandaneto this afternoon and was suggesting I'd do a simple PR to fix the most urgent issue and take a bit more time to fix the eager task creation since this is going to be more complex given how things work atm.
Happy with both :)

let's go with this quick win and iterate over it, the other PR move things around but also includes this fix