RUM-1702: Start session when RUM is initialized

[0xnm]

What does this PR do?

This change starts RUM session during the Rum.enable call. The general rule is like that:

• If application process has foreground type, then session will be started (with interaction time set at the moment of initialization), and ApplicationLaunch view with application_started event will be created.
• Otherwise, if application process has something different from foreground as type, session will be initialized only if background tracking is enabled, no ApplicationLaunch view will be created.

This PR is using the notion of SdkInit class, even though it is possible to use only ApplicationStarted event in order to be on-par with the similar logic in iOS SDK. Also it is still using event system for the session initialization in order to minimize the footprint of the change (it will be easier to rollback it if needed).

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 #1832 (14025c5) into develop (8145782) will decrease coverage by 0.05%.
The diff coverage is 97.67%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1832      +/-   ##
===========================================
- Coverage    83.49%   83.45%   -0.05%     
===========================================
  Files          467      467              
  Lines        16491    16476      -15     
  Branches      2483     2482       -1     
===========================================
- Hits         13769    13749      -20     
- Misses        2043     2045       +2     
- Partials       679      682       +3     

Files	Coverage Δ
...rum/src/main/kotlin/com/datadog/android/rum/Rum.kt	95.56% <100.00%> (+0.82%)	⬆️
...d/rum/internal/domain/scope/RumApplicationScope.kt	90.41% <100.00%> (-4.15%)	⬇️
...g/android/rum/internal/domain/scope/RumRawEvent.kt	100.00% <100.00%> (ø)
...d/rum/internal/domain/scope/RumViewManagerScope.kt	96.99% <ø> (ø)
.../android/rum/internal/monitor/DatadogRumMonitor.kt	83.95% <100.00%> (-0.30%)	⬇️
...droid/rum/internal/domain/scope/RumSessionScope.kt	96.75% <95.45%> (+0.52%)	⬆️

... and 26 files with indirect coverage changes

[fuzzybinary]

This LGTM. Agree we should have @ncreated take a look and make sure the logic is the same.

[ncreated]

It looks okay 👍. Good idea to map the new "sdk init" command into existing "app start" to minimise the change impact 👌.