fix(android): fix null pointer exception at playback start with item metadata

[freeboub]

Summary

Ensure we don't cause nullPointer exception (internally to exoplayer) when starting playback with notification enabled

Motivation

Avoid popup error

Changes

Extracting notification patch from this pull request:
https://github.com/TheWidlarzGroup/react-native-video/pull/3846/files#diff-a35ae7e6ae6efee3b22522a836bcbcdd9d59a214f087df454e4ccbb8cd5c0e76
Changes consist in initializing only once the mediaMetadata during source initialization

Test plan

Can be tested in sample, current issue can be reproduced by going on "no view" with next channel and going back to live channel with channel down

[freeboub]

@KrzysztofMoch please let me know if it makes sens for you please ?

[KrzysztofMoch]

Looks good for me
Only thing is that we are missing hashCode method in Source class (android studio is giving me warning also) I think we can use what android studio is proposing

override fun hashCode(): Int {
    var result = uriString?.hashCode() ?: 0
     result = 31 * result + (uri?.hashCode() ?: 0)
     result = 31 * result + startPositionMs
     result = 31 * result + cropStartMs
     result = 31 * result + cropEndMs
     result = 31 * result + (extension?.hashCode() ?: 0)
     result = 31 * result + (metadata?.hashCode() ?: 0)
     result = 31 * result + headers.hashCode()
     return result
}

WDYT ?