RUM-3597: Start sending batches immediately after feature is initialized

[0xnm]

What does this PR do?

This change removes initial delay for start sending batches, now they the first upload will be triggered right after the feature is initialized instead of waiting for some delay.

This allows to submit more data in case if application is crashing early.

The downside is that there is a chance that first batch uploaded won't be sent to the batch upload metrics telemetry, because RUM monitor may be not ready yet, but this is acceptable.

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 #1991 (8998bda) into develop (71dab01) will increase coverage by 0.10%.
Report is 2 commits behind head on develop.
The diff coverage is 100.00%.

❗ Current head 8998bda differs from pull request most recent head ae36cdc. Consider uploading reports for the commit ae36cdc to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1991      +/-   ##
===========================================
+ Coverage    83.64%   83.74%   +0.10%     
===========================================
  Files          488      488              
  Lines        17771    17773       +2     
  Branches      2666     2666              
===========================================
+ Hits         14863    14883      +20     
+ Misses        2183     2167      -16     
+ Partials       725      723       -2     

Files	Coverage Δ
...in/com/datadog/android/core/internal/SdkFeature.kt	89.10% <100.00%> (+0.07%)	⬆️
...id/core/internal/data/upload/DataUploadRunnable.kt	96.83% <100.00%> (+0.05%)	⬆️
...d/core/internal/data/upload/DataUploadScheduler.kt	100.00% <100.00%> (ø)

... and 24 files with indirect coverage changes