Fix bug 5205275 - SwitchPreference doesn't allow Switch to slide back

and forth.

Also have the SwitchPreference's Switch widget's onClick listener
toggle the switch rather than invoke the preference's own onClick
method. This will allow subclasses to assign different behavior to
clicking the preference itself if desired.

Change-Id: I97e3b7a786e9f0e8bdc03c0ed6001caa7dcd991b