Update UI for factory reset flow

After applying a collapsing toolbar in the Settings app, the factory
reset screen is not displayed correctly. This is because
1) the factory reset page should not essentially have a toolbar since it
has different UI layout form other pages.
2) this page was launched previously as a SubSettings so it applied a
wrong theme.

This change is mainly updating the inheritence structure and specifying
the right theme for the factory reset page in order to have a correct
layout.

Bug: 179457252
Test: robotests and visual verified
Change-Id: I91aec65a120d5a288507d89ace7c9298acdca42b

src/com/android/settings/core/gateway/SettingsGateway.java

@@ -20,6 +20,7 @@ import com.android.settings.AllInOneTetherSettings;
 import com.android.settings.DisplaySettings;
 import com.android.settings.IccLockSettings;
 import com.android.settings.MainClear;
+import com.android.settings.MainClearConfirm;
 import com.android.settings.Settings;
 import com.android.settings.TestingSettings;
 import com.android.settings.TetherSettings;
@@ -282,6 +283,7 @@ public class SettingsGateway {
             WifiAPITest.class.getName(),
             WifiInfo.class.getName(),
             MainClear.class.getName(),
+            MainClearConfirm.class.getName(),
             ResetDashboardFragment.class.getName(),
             NightDisplaySettings.class.getName(),
             ManageDomainUrls.class.getName(),