Fix muting voice call

The index step factor should not be applied when muting a stream that
does not have index min equal to 0. This logic prevented the setting of
index 0 for stream voice call

Flag: EXEMPT bugfix
Test: repro steps in bug
Bug: 362909869
Change-Id: Ibc2db504d2abdc3e5b0c72127566676b5a4cb64a