RUM-3992 Prevent crash in JankStats listener

[xgouchet]

What does this PR do?

Fixes #1976

In Android N (until Android P included), the internal View. findFrameMetricsObserver() method did not check the nullability of the mFrameMetricsObservers field, leading to possible NPE when trying to remove a listener that wasn't yet added.

Motivation

Some customer have seen a crash with an NPE thrown from the Android View when removing the FrameMetricsListener via the JankStats library.
Even though all versions of the official Android View are written in a way that NPE should never happen, those crash still occurred.

[codecov-commenter]

Codecov Report

Merging #1981 (eca3a98) into develop (768fa8a) will decrease coverage by 0.10%.
Report is 1 commits behind head on develop.
The diff coverage is 100.00%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1981      +/-   ##
===========================================
- Coverage    83.71%   83.61%   -0.10%     
===========================================
  Files          487      487              
  Lines        17737    17695      -42     
  Branches      2663     2656       -7     
===========================================
- Hits         14848    14795      -53     
  Misses        2170     2170              
- Partials       719      730      +11     

Files	Coverage Δ
...ernal/vitals/JankStatsActivityLifecycleListener.kt	69.17% <100.00%> (+0.75%)	⬆️

... and 24 files with indirect coverage changes