Bump ExoPlayer to 2.10.6 and OkHttp to 3.12.6

[Redirion]

Little dependency update.
ExoPlayer Changelog: https://github.com/google/ExoPlayer/blob/release-v2/RELEASENOTES.md
OkHttp Changelog: https://square.github.io/okhttp/changelog_3x/#version-3126

• I carefully read the contribution guidelines and agree to them.

[Pentaphon]

That audio focus bugfix on Exoplayer is probably worth merging before the next release.

[Redirion]

It looks like this is more a change for ExoPlayer's own audio focus handling, which NewPipe doesn't use.
Speaking of that I think it might be considerable to make use of ExoPlayer's AudioFocusManager instead of NewPipes own AudioReactor. The AudioReactor might also be a source of "SimpleExoPlayer is accessed from the wrong thread" warnings.