RUMM-2322: Report back interaction as back type, prevent reporting empty target name

[0xnm]

What does this PR do?

This change does the following:

• back interaction are reported as BACK now instead CUSTOM.
• Prevents sending empty target name. It seems it happens because we may start scroll/swipe action with empty target name and without attributes, but onUp is not called, because screen is closed while finger is still touching the screen. To avoid that we will report targetId and targetName for the startUserAction as well, so that once it completes due to timeout, it has the necessary data.

Fixes #964.

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 #980 (350f96f) into develop (5fb1fdf) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #980      +/-   ##
===========================================
- Coverage    83.08%   83.06%   -0.02%     
===========================================
  Files          268      268              
  Lines         9213     9216       +3     
  Branches      1484     1485       +1     
===========================================
+ Hits          7654     7655       +1     
- Misses        1151     1153       +2     
  Partials       408      408              

Impacted Files	Coverage Δ
...ernal/instrumentation/gestures/GesturesListener.kt	93.08% <100.00%> (+0.16%)	⬆️
.../instrumentation/gestures/WindowCallbackWrapper.kt	92.31% <100.00%> (ø)
...ndroid/core/internal/persistence/file/EventMeta.kt	80.00% <0.00%> (-10.00%)	⬇️
...android/rum/internal/ndk/DatadogNdkCrashHandler.kt	87.50% <0.00%> (-0.54%)	⬇️
...rc/main/java/com/datadog/opentracing/DDTracer.java	55.65% <0.00%> (-0.42%)	⬇️
...droid/rum/tracking/FragmentViewTrackingStrategy.kt	86.96% <0.00%> (+4.35%)	⬆️

[xgouchet]

This interaction is handleBackEvent, and there's a BACK type for actions, maybe we should use that instead as it more precise

[0xnm]

thanks! I didn't know back type exists in the RUM schema. I've updated my PR.