Implement audio focus as per android docs:
https://developer.android.com/guide/topics/media-apps/audio-focus
https://medium.com/androiddevelopers/audio-focus-3-cdc09da9c122

AUDIOFOCUS_LOSS should abandon focus and not try resuming audio, this is done with AUDIOFOCUS_LOSS_TRANSIENT

This fixes at least:
- Audio not being paused after focus being taken by some voip applications
- Content resuming and pausing instantly sporadically (some race condition perhaps) when activity was resumed from background.

Fix playerviewcontroller keypath leak of observers

The progress message handler will duplicate recursions of the `onProgressMessage` handler on change of player state from any state to STATE_READY with `playWhenReady` is true (when video is not paused). This clears the messages on STATE_IDLE and STATE_BUFFERING to break the recursion.

fix typo in README

- Bump Exoplayer to 2.10.4
- Remove deprecated usages of Exoplayer methdos
- Add `ReactExoplayerConfig` as extension points to configure the Exoplayer instance

Fixed Exoplayer doesn't work with mute=true #1696

Added support for automaticallyWaitsToMinimizeStalling property on iOS 

Removing the call to observeValueForKeyPath:ofObject:change:context: …

Updated README.md to include instructions for React Native 0.60 and a…

[FIX] Fix release 5.0.0 bad merge with 5.0.1

fix crash iOS9

If the super class is not actually observing the key, the app will crash. Checking to see if
the super class responds to this selector doesn't solve this issue.

react-native-video github issue: https://github.com/react-native-community/react-native-video/issues/1515

Discussion about this particular problem: https://stackoverflow.com/questions/6574714/whats-wrong-with-this-observevalueforkeypathofobjectchangecontext-implement