ExoPlayer FFmpeg extension Build doc update

[Nodrex]

In build doc (https://github.com/google/ExoPlayer/tree/release-v2/extensions/ffmpeg)
there is written that supported NDK is r20, as I know this version of NDk is deprecated, because there is r22 already.
So is this doc outdated and needs a refresh or we need to downgrade to deprecated r20 NDK to build FFmpeg extension ?

[kim-vde]

Thanks for reporting this.

Android NDK r20 is indeed deprecated. I tested the extensions with r22 and it seems to be working. I will update the documentation.

[Tolriq]

@kim-vde NDK 21 is the LTS version and should be used. NDK 22 have possible big size regressions and should be full tested against that.

[kim-vde]

Yes, you are right. It is better to use the latest LTS version because it is supported for longer (I don't know about the regressions in r22). I updated the guidelines.