RUM-520 read DATADOG_API_KEY from gradle properties

[xgouchet]

What does this PR do?

Ensure we read the API Key from all documented sources Fixes #184

Motivation

What inspired you to submit this pull request?

Additional Notes

Anything else we should know when reviewing?

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 #186 (10e9642) into develop (47979b8) will increase coverage by 0.17%. The diff coverage is 75.00%.

@@              Coverage Diff              @@
##             develop     #186      +/-   ##
=============================================
+ Coverage      86.63%   86.81%   +0.17%     
+ Complexity       178      177       -1     
=============================================
  Files             21       21              
  Lines            576      576              
  Branches          72       75       +3     
=============================================
+ Hits             499      500       +1     
  Misses            49       49              
+ Partials          28       27       -1     

Files Changed	Coverage Δ
...com/datadog/gradle/plugin/DdAndroidGradlePlugin.kt	75.21% <75.00%> (+0.85%)	:arrow_up: