Make footer preferences non-clickable by default

Both the footer preference and the TextView they contain
were clickable, which confused some accessibility services.

Bug: 38511461
Test: Verified that switch access no longer offers users
the option of clicking on the footer for accessibility
services and Magnification as well as in the Battery
settings screen.

Change-Id: I937340fda93f6a8097919e1f86d5d890487370bc