Set preference unselectable using the DividerPreference.

In this way, when talking back is on, device won't speaking "Double
tap to activate" for no-action perference. The preference will have
divider line but lose ripple animation.

Bug: 31051833
Test: Manual Test. Cannot find a way to test it in testing framework.
1. Turn on the talk back.
2. Go to "About phone" Settings.
3. Tap and listen.

Change-Id: I2066d574e14071408c5a1db85a3bd4dafd8429fe