Fix ringer mode being unneededly changed by setting ringer mode volume to 0.

When having set the ringer mode to silent on a device with vibrator and
calling setStreamVolume() for the ringer stream again (e.g. in one of
the volume control dialogs in the settings app), the ringer mode is set
to vibrate, although there's no reason to do so.

Fix that by remembering the last non-normal (that is, either silent or
vibrate) ringer mode and using that when setting the stream volume to 0.

Change-Id: Idb4a34743c0db5da61b3354889212fbdc9e3ee81
JIRA: CYAN-1403