Flush RN UI queue on Android

[tomduncalf]

What, How & Why?

This makes the tests run (almost) reliably on Android. I get 5 failures but for seemingly unrelated reasons. Previously many of the roundtrip tests would fail at random.

Note: it seems that the JNI header generator default has changed jclass to jobject. I just went with it but I'm not sure if this might cause issues!

Fixes #4389 for Android (already fixed on iOS)

☑️ 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

[takameyer]

I can't say much about this code, but I tried the branch out and the tests run much better now.

[fronck]

LGTM