Fix Settings Search for OneHandedSettings

- Marks top header preference as non-searchable
- Makes shortcut preference searchable

Fix: 353591062
Test: manually confirm above behaviors
Test: atest OneHandedSettingsTest
Flag: com.android.settings.accessibility.fix_a11y_settings_search
Change-Id: I3355f817358cec1d265b89d75229ffc2742efe1c