ios only: sound not playing when app is in background and some other app has sound on (#369)

Platforms affected

ios

Motivation and Context

The audio would not start if app is in background and some other app audio is running.
Reported issue:
#369

Description

update audio session with withOptions:AVAudioSessionCategoryOptionMixWithOthers so that we can play audio on top of other audio when our app is in background

Testing

I have tested the changes on my ios app.

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary