RUM-1017: Unregister RUM monitor when associated RUM feature is stopped

[0xnm]

What does this PR do?

This change removes RUM monitor for the given RUM feature (or SDK instance in other words), when the feature is stopped.

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 #1725 (ae0408f) into develop (50e6072) will decrease coverage by 0.02%.
Report is 8 commits behind head on develop.
The diff coverage is 81.25%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1725      +/-   ##
===========================================
- Coverage    83.41%   83.38%   -0.02%     
===========================================
  Files          465      465              
  Lines        16092    16093       +1     
  Branches      2386     2385       -1     
===========================================
- Hits         13422    13419       -3     
- Misses        2025     2035      +10     
+ Partials       645      639       -6     

Files	Coverage Δ
...lin/com/datadog/android/rum/internal/RumFeature.kt	92.21% <100.00%> (+0.35%)	⬆️
...kotlin/com/datadog/android/rum/GlobalRumMonitor.kt	81.25% <80.00%> (-1.51%)	⬇️

... and 21 files with indirect coverage changes