AudioService: removed last audible volume index

Removed storage of last audible stream volume index.
This information is redundant because we already have
two pieces of information (current volume and mute state)
allowing to completely define which volume should actually
be applied by the framework or indicated to apps.

The last audible index management had anyway been broken by
successive changes making that the value stored could be 0
in some cases causing problems like issue 8109149 when
restoring the volume settings from the DB.

Bug 8109149

Change-Id: Iea80f5a994554e42d45b227f651ec9646844aa77