Add content description to the master switch toggle button.

Set the preference title as the toggle button's content description, so
that when user focus on the button, it will read the lable as well on
top of just "switch on" or "switch off".

Change-Id: Id1954fd54b7d38cb4e75d8b3e49d05ee694e0e41
Fix: 37618855
Test: make RunSettingsRoboTests