fix(ios): Configure audio session before activation and only if playing

[soutua]

Summary

There's an issue on iOS that even if the player is configured to mix audio with others and even if the player is muted, when the player is mounted it steals audio focus from possible background audio.

Motivation

Fixes #4610

We've been running this change as a patch in our production app for a couple of months with no issues and it fixed the linked issue.

Changes

Configuring the audio session only if there's an existing player and configuring the audio session before activating it.

The idea in the fix is that we do not even try to configure the audio session if there there are no players playing, since it's not possible to configure it correctly unless there are some existing players where we would get the configuration values from, so it ends up defaulting in possibly incorrect audio category/mode in that case. Also do the configuration before activating the audio session since that's what Apple recommends:
"Typically, you set the category and mode before activating the session. You can also set the category or mode while the session is active, but doing so results in an immediate change." from: https://developer.apple.com/documentation/avfaudio/avaudiosession/setcategory(_:mode:options:)

Test plan

• Start a background audio in another app (e.g. a music player)
• Start the app with the react-native-video player that is configured to mix audio with others or is muted
• The background audio should no longer stop playing when the app is started