Use RumFeature#context instead of CoreFeature#contextRef in RumFeature

[0xnm]

What does this PR do?

Tiny change to use RumFeature#context which is set during the initialization instead of CoreFeature#contextRef: WeakReference(which may disappear).

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 #982 (ef707b2) into feature/sdkv2 (384e618) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@                Coverage Diff                @@
##           feature/sdkv2     #982      +/-   ##
=================================================
- Coverage          82.67%   82.66%   -0.01%     
=================================================
  Files                296      296              
  Lines               9777     9777              
  Branches            1617     1617              
=================================================
- Hits                8083     8082       -1     
- Misses              1210     1211       +1     
  Partials             484      484              

Impacted Files	Coverage Δ
...lin/com/datadog/android/rum/internal/RumFeature.kt	92.16% <100.00%> (ø)
...rsistence/file/batch/PlainBatchFileReaderWriter.kt	86.27% <0.00%> (-4.90%)	⬇️
...ndroid/telemetry/internal/TelemetryEventHandler.kt	90.32% <0.00%> (-1.61%)	⬇️
...android/rum/internal/ndk/DatadogNdkCrashHandler.kt	86.70% <0.00%> (-0.53%)	⬇️
...rc/main/java/com/datadog/opentracing/DDTracer.java	56.07% <0.00%> (ø)
...ain/java/com/datadog/opentracing/PendingTrace.java	60.34% <0.00%> (+1.72%)	⬆️
...tadog/android/core/internal/net/CurlInterceptor.kt	83.33% <0.00%> (+2.38%)	⬆️
...g/internal/domain/event/DdSpanToSpanEventMapper.kt	100.00% <0.00%> (+4.08%)	⬆️
...android/log/internal/logger/TelemetryLogHandler.kt	100.00% <0.00%> (+25.00%)	⬆️