[jni_flutter] Fix release mode crash

[liamappelbe]

Add an example so I can repro release mode missing symbols bug, then update the proguard rules to fix it.

Fix #3280

[github-actions]

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[dcharkes]

I am assuming the example app is just the generated stuff from flutter create? I didn't review that.

[liamappelbe]

I am assuming the example app is just the generated stuff from flutter create? I didn't review that.

Yeah pretty much. I just updated lib/main.dart to invoke the package's APIs