Investigate how to improve threading model

[nt4f04uNd]

Current synchrounous nature of ContentProvider does not get along well with Flutter channel threading model, where all the messaging happens synchronously

This have proven to lead bugs like this one
ryanheise/audio_service#1063

Code
https://github.com/nt4f04uNd/android_content_provider/blob/master/android/src/main/kotlin/com/nt4f04und/android_content_provider/SynchronousMethodChannel.kt