Change condition to enter silent mode.

Previously, silent mode was entered if volume was already at 0 and
volume down key was pressed once.

The new volume management policy specifies that silent mode is entered when
volume is at last audible step and volume down key is pressed once.

Pressing volume down key continuously will decrease volume until
last audible step is reached, not 0.

Change-Id: Ib7b530668473d6a5af1e748f445d6185afed2158