[RUM 11204] [Modularization] Use native SDK's core instance instead of DatadogSdkWrapperStorage's

[sbarrio]

What does this PR do?

This PR makes it so the SDK relies on Datadog.getInstance() to get the native SDK core instance instead of keeping a reference to it inside DatadogSdkWrapperStorage.

Motivation

This is part of the modularization effort being done for v3.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests
• 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)
• If this PR is auto-generated, please make sure also to manually update the code related to the change

[marco-saia-datadog]

Tests are currently failing on Android:

e: file:///Users/ec2-user/builds/JBp7x9hf6/0/DataDog/dd-sdk-reactnative/packages/internal-testing-tools/android/src/test/kotlin/com/datadog/reactnative/internaltesting/DdInternalTestingImplementationTest.kt:75:27 Unresolved reference: Datadog
> Task :compileDebugUnitTestKotlin FAILED
e: file:///Users/ec2-user/builds/JBp7x9hf6/0/DataDog/dd-sdk-reactnative/packages/internal-testing-tools/android/src/test/kotlin/com/datadog/reactnative/internaltesting/DdInternalTestingImplementationTest.kt:75:27 Unresolved reference: Datadog
e: /Users/ec2-user/.gradle/caches/transforms-3/855f9b1d925aa44010257b44e8aa35f7/transformed/react-android-0.76.9-debug/jars/classes.jar!/META-INF/ReactAndroid_debug.kotlin_module: Module was compiled with an incompatible version of Kotlin. The binary version of its metadata is 1.9.0, expected version is 1.6.0.