Ignore list item clicks if PreferenceActivity is paused.

Since list item clicks are delivered asynchronously, it's possible to
get click callbacks after the activity has already paused.

This should actually be blocked further up the event chain, ideally,
so that the list selection state is not out of sync with the application.

Bug: 9752185
Change-Id: I1058d64cba98e9093b462e2e581bdbccdace1420