fix(android): Avoid calling onDuration on position event (closes #136)

[Gustl22]

Description

Unlike on other platforms, onDuration is constantly called on Android. This PR avoids calling onDuration on every position event.
OnDuration is still once called with the correct duration after having prepared the source.

Checklist

• 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.

Related Issues

Closes #136