RUM-4031: Avoid eager fetching of Variant values

[0xnm]

What does this PR do?

This PR switches to the lazy configuration of upload task by using TaskContainer#register API instead of TaskContainer#create. This solves the problem of compatibility with https://github.com/Triple-T/gradle-play-publisher plugin which is similar to what described here Triple-T/gradle-play-publisher#940.

Long-term solution will be to switch from the legacy Variant API to the new Variant API.

Note: This is a partial fix, because if both tasks try to configure themselves in the same build execution, the problem will still occur. But it is not clear if it should be fixed on our side or problem is on the gradle-play-publisher plugin side.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)

[codecov-commenter]

Codecov Report

Merging #240 (2651409) into release/1.13.0 (85c7d67) will increase coverage by 0.57%.
The diff coverage is 83.33%.

Additional details and impacted files

@@                 Coverage Diff                  @@
##             release/1.13.0     #240      +/-   ##
====================================================
+ Coverage             76.65%   77.22%   +0.57%     
  Complexity              201      201              
====================================================
  Files                    26       26              
  Lines                   788      790       +2     
  Branches                104      104              
====================================================
+ Hits                    604      610       +6     
+ Misses                  147      143       -4     
  Partials                 37       37              

Files	Coverage Δ
...tlin/com/datadog/gradle/plugin/DdFileUploadTask.kt	80.65% <100.00%> (-0.66%)	⬇️
...com/datadog/gradle/plugin/DdAndroidGradlePlugin.kt	61.11% <80.77%> (+0.24%)	⬆️

... and 1 file with indirect coverage changes