Fix issue 2589563: Able to set ringer volume to 0 outside of silent mode.

The problem happens only if volume down key is pressed while
already in silent or vibrate mode. In this case, AudioService accepts
changing the last audible index for ringer volume from 1 to 0.
When volume up is pressed, current ringer volume value is restored from 0
instead of 1.

The fix consists in keeping last audible index at 1 once in silent of vibrate
mode even is volume down key is pressed.

Change-Id: Id08edd12ce985c22233124545eba2c3e4b8cdc8e