[RUM-5566] Added setSyntheticsAttribute in RumInternalProxy

[marco-saia-datadog]

What does this PR do?

Adds setSyntheticsAttribute in _RumInternalProxy for internal usage in cross-platform frameworks.

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)

[marco-saia-datadog]

I have added a check to verify that the lateinit variable sdkCore has been initialized in the ActivityLifecycleTrackingStrategy:

if (::sdkCore.isInitialized) {
/// ...
}

A test was failing because of this missing check, as the previous implementation would only use the sdkCore if the activity contained the synthetics attributes intent extras, that are not passed for that particular test.

@mariusc83 @xgouchet let me know if you prefer to modify the test instead, or if you think this approach is safer.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 75.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 69.97%. Comparing base (b9848cc) to head (13008c3).

Files	Patch %	Lines
...otlin/com/datadog/android/rum/_RumInternalProxy.kt	75.00%	0 Missing and 2 partials ⚠️
.../rum/tracking/ActivityLifecycleTrackingStrategy.kt	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2133      +/-   ##
===========================================
- Coverage    69.99%   69.97%   -0.02%     
===========================================
  Files          726      726              
  Lines        26984    26995      +11     
  Branches      4520     4524       +4     
===========================================
+ Hits         18887    18889       +2     
- Misses        6827     6828       +1     
- Partials      1270     1278       +8     

Files	Coverage Δ
.../rum/tracking/ActivityLifecycleTrackingStrategy.kt	90.00% <75.00%> (+2.73%)	⬆️
...otlin/com/datadog/android/rum/_RumInternalProxy.kt	78.57% <75.00%> (-4.76%)	⬇️

... and 29 files with indirect coverage changes