Merge voice and timer options into a single "no-hands shutter" option

... and add a first-use hint to VS listing the hotwords

Usage of handsfree modes can be made continuous by flipping a switch
in the settings pane. The default behavior is to reset the shutter
mode to normal after a picture is taken

Change-Id: I271c7ba71f320e19be5d1a6a8d98dafeb4b8385a