Further rework of the Android module

In my opinion, we should separate the recognition process and the audio recording process.

## Motivation
There are already a bunch of Flutter plugins for recording from a microphone(https://pub.dev/packages?q=microphone) and it would be strange to reinvent/reimplement them in the vosk-flutter plugin. Additionally, it would be great to unite the logic for all platforms instead of having the MethodChannel implementation for Android and the FFI implementation for other platforms.

## Plan
The plan is to remove the dependency to `vosk-android` in the Android module and replace the MethodChannel with FFI.
Details:
- we should put Android binaries somewhere, maybe add them to the Releases of the vosk-api repo
- we should rewrite `SpeechService` from the `vosk-android` in Dart and it should just recieve a stream of data instead of the microphone recording

@nshmyrev what do you think about this?