Fix incorrect UI after triple-tap is disabled

The preference summary is not changed after
users disable the triple tap shortcut by the confirm
dialog. It is due to the fact that the fragment doesn't
observe the settings value change.

To fix it, we add the settings key to observe the change.

Bug: 208755360
Test: manual test
Change-Id: Ie5b7fc5e67c5491b53485af0d3d68d1789aeace4