Fix issue 2592680: Saved ringer volume forced to 0 when receiving a call in silent mode.

This is a regression introduced by change 5b4e654d for issue 2472495.
When AudioService changes audio mode, setMode() reapplies current volume for the default active stream
which in this case is STREAM_RING.
Because the new implementation of silent mode actually mutes the ringer stream,
setStreamVolumeInt() now applies the volume change received while in silent mode
to the last audible value and we end up clearing the last audible volume for ringer.

The fix consists in not modifying last audible value when the new value is 0.

Also removed obsolete code in setStreamVolumeInt() since new implementation of setRingerModeInt()
in change 5b4e654d.

Change-Id: I746f3bc1af39a602ce12d130ce592007b2d0ebb6