RUM-3430: Fix application startup time regression

[0xnm]

What does this PR do?

Fixes #1899.

The change in #1832 broke the application startup time measurement (which was bound to the first RUM event before), because with this change application startup time was dependent on the SDK init moment, which is not the correct way to measure it.

This change brings old behaviour back by ignoring SDK init event.

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 #1935 (8b59811) into develop (684f5b4) will increase coverage by 0.02%.
Report is 14 commits behind head on develop.
The diff coverage is 95.65%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1935      +/-   ##
===========================================
+ Coverage    83.31%   83.33%   +0.02%     
===========================================
  Files          481      481              
  Lines        17412    17413       +1     
  Branches      2588     2591       +3     
===========================================
+ Hits         14506    14510       +4     
- Misses        2195     2196       +1     
+ Partials       711      707       -4     

Files	Coverage Δ
...d/rum/internal/domain/scope/RumApplicationScope.kt	93.48% <100.00%> (+3.07%)	⬆️
...g/android/rum/internal/domain/scope/RumRawEvent.kt	100.00% <ø> (ø)
.../android/rum/internal/monitor/DatadogRumMonitor.kt	84.84% <100.00%> (-0.42%)	⬇️
...droid/rum/internal/domain/scope/RumSessionScope.kt	96.30% <50.00%> (-0.45%)	⬇️

... and 27 files with indirect coverage changes