Improve documentation on routing of volume key events to active MediaSession

### Issue description
When playing content using the CastPlayer, the device's physical volume buttons should control the receiver device's volume. This behaves as expected when the CastPlayer is contained within an Activity. However, when the CastPlayer is contained within a Service (such as a MediaBrowserServiceCompat) the physical volume buttons change the sender device's media volume.

### Reproduction steps
I've created 2 sample projects:

- [CastVolumeControlTest](https://github.com/deRonbrown/CastVolumeControlTest) shows the proper volume control stream behavior while casting.
- [CastVolumeControlServiceTest](https://github.com/deRonbrown/CastVolumeControlServiceTest) shows the incorrect volume control stream behavior while casting.

### Link to test content
Test mp4 file is used in sample projects above.

### Version of ExoPlayer being used
Core 2.8.4 and Cast Extension 2.8.4

### Device(s) and version(s) of Android being used
Reproduces on 9.0 and 8.1.0.