Settings: safe headset volume option (2 of 2)

difference to CM and our old implementation added a warning
dialog. A bit extended due that we will have in future other
warning dialogs in soundsettings as well

Change-Id: I6546403484fd773abd9560a8405d75bb682f8b62

Conflicts:
	res/values/slim_strings.xml
	src/com/android/settings/slim/SoundSettings.java