RUM-3670 use backpressured executors

[xgouchet]

What does this PR do?

Apply the Backpressure strategy and blocking queue to all ExecutorServices.

Additional Notes

Note

We are still using our legacy LoggingScheduledThreadPoolExecutor for scheduled tasks (e.g.: uploader, vitals tracking, …). Those will only have one our two tasks in their queue, so it's not as urgent to use the backpressure strategy there. A task (RUM-3704) has been created to keep track of this.

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 #1939 (19454fc) into release/2.7.0 (c9243d1) will decrease coverage by 0.25%.
The diff coverage is 50.60%.

Additional details and impacted files

@@                Coverage Diff                @@
##           release/2.7.0    #1939      +/-   ##
=================================================
- Coverage          83.37%   83.13%   -0.25%     
=================================================
  Files                485      484       -1     
  Lines              17497    17529      +32     
  Branches            2598     2591       -7     
=================================================
- Hits               14588    14571      -17     
- Misses              2203     2256      +53     
+ Partials             706      702       -4     

Files	Coverage Δ
.../datadog/android/core/internal/thread/ThreadExt.kt	60.47% <ø> (ø)
...rum/src/main/kotlin/com/datadog/android/rum/Rum.kt	91.84% <100.00%> (+0.35%)	⬆️
...lin/com/datadog/android/rum/internal/RumFeature.kt	92.99% <100.00%> (+0.30%)	⬆️
.../android/rum/internal/monitor/DatadogRumMonitor.kt	87.26% <100.00%> (+1.59%)	⬆️
...nal/tracking/AndroidXFragmentLifecycleCallbacks.kt	94.29% <100.00%> (-0.16%)	⬇️
...nternal/tracking/OreoFragmentLifecycleCallbacks.kt	89.47% <100.00%> (-0.27%)	⬇️
.../rum/tracking/ActivityLifecycleTrackingStrategy.kt	86.96% <100.00%> (+4.73%)	⬆️
...droid/rum/tracking/ActivityViewTrackingStrategy.kt	90.00% <100.00%> (+2.90%)	⬆️
...ain/kotlin/com/datadog/android/core/DatadogCore.kt	83.85% <77.78%> (-0.64%)	⬇️
...n/com/datadog/android/core/internal/CoreFeature.kt	86.69% <86.36%> (-0.48%)	⬇️
... and 2 more

... and 28 files with indirect coverage changes

[0xnm]

I covered so far only production files in a quick look

[0xnm]

lgtm. I left some comments/suggestions, but they are not blocking.

[mariusc83]

LGTM But I would really like us to think about what I added there as a opened question.