Remove Runtime shutdown hook when SDK instance is stopped

[0xnm]

What does this PR do?

Otherwise it is possible to have a memory leak when SDK instance is stopped, because reference to the SDK instance still exists through the hook runnable.

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 #1956 (fac1b96) into develop (0f0d085) will decrease coverage by 0.05%.
The diff coverage is 55.56%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1956      +/-   ##
===========================================
- Coverage    83.21%   83.16%   -0.05%     
===========================================
  Files          483      483              
  Lines        17584    17566      -18     
  Branches      2605     2603       -2     
===========================================
- Hits         14632    14608      -24     
+ Misses        2257     2254       -3     
- Partials       695      704       +9     

Files	Coverage Δ
...ain/kotlin/com/datadog/android/core/DatadogCore.kt	81.25% <55.56%> (-2.60%)	⬇️

... and 30 files with indirect coverage changes