Replace the video_player plugin with just_audio for playing audio

[llucax]

It turns out that this plugin doesn't support audio officially (flutter/flutter#38480 (comment), flutter/flutter#75636). It doesn't work on iOS and if there is any issue in the platform where it does work, there will be no effort spent in fixing it.

Because of this we should switch to another package.

There are several audio packages out there, but all comes with different limitations, which makes the decision of what to use a bit difficult.

just_audio seems like the winner, but here is a list of packages that seem to be actively maintained:

• https://pub.dev/packages/audioplayers
  Popularity: 98% popularity
  Android support: MediaPlayer
  Main limitations:

  • It can't play assets in web
  • Audio playback can't be controlled in web
  • Ugly/awkward interface for playing asserts
  • Using LOW_LATENCY option throws weird errors
  • Media Player comes with Android and device-specific issues can't be addressed.

  Highlights: It doesn't suffer from ExoPlayer bug when playing short audios (Short sounds are not played if playing is done a few seconds after initialization #15)

• https://pub.dev/packages/just_audio
  Popularity: 96%
  Android support: ExoPlayer
  Main limitations: Suffers from ExoPlayer bug when playing short audios (Short sounds are not played if playing is done a few seconds after initialization #15)
  Highlights: Good architecture, well thought, advance and nice interface.

• https://pub.dev/packages/soundpool
  Popularity: 93%
  Android support: Sound Pool
  Main limitations: Some features are Android-only, can't tell when an audio finished playing
  Highlights: Very low latency, is mainly useful to use it for games or notification sounds or sounds that are used many times repeatedly

• https://pub.dev/packages/flutter_sound
  Popularity: 93%
  Android support: MediaPlayer
  Main limitations: (L)GPL license (unclear because is not clear what the Tau project is), strange documentation, heavyweight (it's a full sound framework including audio recording)
  Highlights: It doesn't suffer from ExoPlayer bug when playing short audios (Short sounds are not played if playing is done a few seconds after initialization #15)

[llucax]

Blocked by google/ExoPlayer#8569.

[llucax]

We should definitely use just_audio as it is now marked as a Flutter Favorite.