Do not request audio focus if the played video has no audio

Use case description

When setting up exoplayer by calling ExoPlayer.setAudioAttributes(...) with handleAudioFocus set to true, the audio focus is always requested even when the video file contains no audio tracks or all audio tracks have been disabled.

This means that the burden of deciding whether the audio focus should really be requested or not is on the application, which would then need to inspect the content of the video file.

Proposed solution

When handleAudioFocus==true, Exoplayer should NOT actually request the audio focus when either the video contains no audio tracks or when all audio tracks have been disabled.

Alternatives considered

Rename handleAudioFocus argument to requestAudioFocus to at least make expectations clearer.