Recursively remove preference from screen.

The preference framework does not remove pref recursively. So when a
preference is nested in preference hierarchy, it's not safe to simply
call screen.removePreference(). We need to first find its parent and
remove pref from its parent.

Change-Id: Ic7fefa498ed71a8877d862845ddcc2d6d6034a55
Fix: 38507066
Test: make RunSettingsRoboTests