media volume: remove old --set and rename --index to --set

The old --set did not take a parameter for volume and was
defaulting to volume index of 5.
The old --index option allowed the user to set the volume to
a specified value.
This CL removes the old --set that had limited usefulness, and
renames --index to --set, so --set now takes a parameter for
volume.
For backward compatibility the --index option is kept around
to allow scripts time to switch to --set

Bug: 33791633
Test: adb shell media volume --set 4
Change-Id: I43a87a878439b3416efa746708b62a869023acdc