Remove nested preference from its parent.

PreferenceScreen.remvoePreference() only removes top level preference in
the preference screen. When we try to remove preference inside a
preference category, it will fail. Add handling to recursively find
the preference and remove it directly from its parent instead.

Change-Id: Ib33efe0b716db2366d712ce1d1eb8b5e33a4683a
Fix: 35365702
Test: make RunSettingsLibRoboTests