[Fix] Externalize setting of minSdkVersion on sdk Android packages

[sbarrio]

What does this PR do?

This PR modifies the SDK android packages build.gradle files so minSdkVersion is set to follow the version indicated by the root package including it instead of being forced to API level 21.

This change was proposed by @nbennink at #867. Thank you!

Motivation

The sdk should be flexible enough to adapt to its host application sdk target settings.

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

[datadog-datadog-prod-us1]

Datadog Summary

✅ Code Quality    ✅ Code Security    ✅ Dependencies

Test Optimization Report

Branch report: sbarrio/fix/allow-external-setting-of-minsdkversion
Commit report: d2967e4
Test service: dd-sdk-reactnative

✅ 0 Failed, 669 Passed, 1 Skipped, 3.78s Total Time

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