Add fix for older Android Gradle plugin

[tomduncalf]

What, How & Why?

#4697 inadvertently changed the minimum Android Gradle plugin version required when using Realm to 4.2.x. This change fixes that by wrapping the code in question in a version check, and adding a fallback to the old syntax for older Android Gradle versions (probably not strictly necessary but this would allow us to debug apps using the older Android Gradle version without modification).

I also reverted the test app back to the old version, so that we are still testing that version on CI

Fixes #4706

☑️ ToDos

• 📝 Changelog entry
• 📝 Compatibility label is updated or copied from previous entry
• 🚦 Tests
• 🔀 Executed flexible sync tests locally if modifying flexible sync
• 📦 Updated internal package version in consuming package.jsons (if updating internal packages)
• 📱 Check the React Native/other sample apps work if necessary
• 📝 Public documentation PR created or is not necessary
• 💥 Breaking label has been applied or is not necessary

If this PR adds or changes public API's:

• typescript definitions file is updated
• jsdoc files updated
• Chrome debug API is updated if API is available on React Native