fix(android): Allow AudioFocus.none

[Gustl22]

Description

• fix(android): allow AndroidAudioFocus.none (closes Android: impossible to have non-ducking, non-interrupting playback of other audio when AudioContext is used #1495)
• refactor: values for AudioContext enums
• refactor: change default behavior to use AndroidAudioFocus.gain, see https://developer.android.com/media/optimize/audio-focus

Checklist

• Need to test against Android API <26, probably compatible with API 19
• The title of my PR starts with a Conventional Commit prefix (fix:, feat:, docs:, chore: etc).
• I have read the Contributor Guide and followed the process outlined for submitting PRs.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation and added dartdoc comments with ///, where necessary.
• I have updated/added relevant examples in example.

Breaking Change

• Yes, this is a breaking change.
• No, this is not a breaking change.

Migration instructions

The AndroidAudioFocus is now by default set to gain.
To change the behavior set the AndroidAudioFocus to none:

  AudioPlayer.global.setGlobalAudioContext(
    AudioContext(
      android: AudioContextAndroid(
        audioFocus: AndroidAudioFocus.none,
      ),
    ),
  );

Related Issues

Closes #1495
Closes #1534