[pigeon] Adds Kotlin lint tests to example code and fix lints

[bparrishMines]

Fixes flutter/flutter#166780

Also moves Kotlin utility methods into a private object so that they don't trigger the SyntheticAccessor lint.

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[stuartmorgan-g]

Can we add the lint that found this to the Pigeon test harness plugin's example app?

[bparrishMines]

This fixes the GradleDynamicVersion lint.

[bparrishMines]

@tarrinneal @stuartmorgan-g I could only get ImplicitSamInstance lint to fail on ./gradlew lint but not on build. I updated the custom tests of the plugin to run lint on test_plugin. It seems that ImplicitSamInstance is already included, but calling handler.removeCallbacks { this.releaseAllFinalizedInstances() } doesn't seem to trigger it. But calling handler.removeCallbacks(::releaseAllFinalizedInstances) does.

[stuartmorgan-g]

LGTM once it's added to the non-CI runner logic, per comment.