Make constructors of DatadogSite private

[0xnm]

What does this PR do?

Constructors of DatadogSite shouldn't be public.

This may be see as a breaking change (removing items from public API), but this was never intended to be public.

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

All modified and coverable lines are covered by tests ✅

Project coverage is 83.77%. Comparing base (cff8b54) to head (9354705).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2010      +/-   ##
===========================================
+ Coverage    83.71%   83.77%   +0.06%     
===========================================
  Files          488      488              
  Lines        17736    17767      +31     
  Branches      2659     2666       +7     
===========================================
+ Hits         14846    14883      +37     
+ Misses        2177     2169       -8     
- Partials       713      715       +2     

Files	Coverage Δ
...src/main/kotlin/com/datadog/android/DatadogSite.kt	100.00% <100.00%> (ø)

... and 30 files with indirect coverage changes

[xgouchet]

That works, but it isn't an issue, as enum have a private init.
If someone tried to write the following, it wouldn't compile anyway :

val customSite = DatadogSite("custom", "custom-intake.example.com")

This line triggers two compilation errors:

• Cannot access '<init>': it is private in 'DatadogSite'
• Enum types cannot be instantiated

[0xnm]

@xgouchet yes, my concern was mostly the look of the API surface file, these custom constructors are declared there.