RUMM-3356 fix jankstats

[xgouchet]

What does this PR do?

This PR aims at solving some issues found when using the JankStats tool to track Mobile Vitals

• Delay the initialization to onActivityStarted(): before that the window is not fully created and the JankStats creation would often fail
• Add some tracking of the active windows: several activities could theoretically share the same window. This PR tracks this to avoid registering more than one listener to the same window
• Furthermore, we now listen to onActivityStopped() to prevent keeping a listener on a Window not being used anymore.

Additional Notes

• Since the JankStats doesn't measure the actual frame rate, but the time needed to draw the frame, scaling by the device's maximum frame rate doesn't make sense anymore.
• A custom view in the sample app is added to illustrate what a slow framerate can look like; it uses a random image generation (avoids possible optimization by the compiler or JIT), and performs a lot of bitmap operation that makes it very slow.

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 #1512 (d22aeed) into develop (a982b35) will decrease coverage by 0.04%.
The diff coverage is 80.36%.

@@             Coverage Diff             @@
##           develop    #1512      +/-   ##
===========================================
- Coverage    82.00%   81.96%   -0.04%     
===========================================
  Files          363      364       +1     
  Lines        12981    13018      +37     
  Branches      2162     2168       +6     
===========================================
+ Hits         10645    10670      +25     
- Misses        1679     1685       +6     
- Partials       657      663       +6     

Impacted Files	Coverage Δ
...lin/com/datadog/android/rum/internal/RumFeature.kt	92.48% <ø> (-1.55%)	⬇️
...g/android/rum/internal/vitals/JankStatsProvider.kt	10.00% <10.00%> (ø)
...ernal/vitals/JankStatsActivityLifecycleListener.kt	92.45% <95.65%> (+12.45%)	⬆️

... and 14 files with indirect coverage changes

[0xnm]

nice work, thanks for the help! I added few comments/suggestions.

[0xnm]

Suggested change

	fun `𝕄 foNothing 𝕎 onActivityStopped() {}`() {
	fun `𝕄 do nothing 𝕎 onActivityStopped() {}`() {

[0xnm]

probably should be placed below, around line 673.

[0xnm]

I'm wondering if we should use WeakHashMap for activeActivities and activeWindowsListener, in case if the window was temporary and will be destroyed once a particular activity is destroyed.

I can see that PolicyManager.makeNewWindow can be called from different places, Dialog constructor included. So if there is a dialog shown and then destroyed, we will be keeping Window instance in the memory forever, preventing it from be GCed.

[0xnm]

lgtm!