SUW: use standard navigation method

Use software/hardware back button and add on-screen Next & Skip buttons.

Change-Id: I7bfa052f2217e51ea778df688e7169657aaad2c6

src/org/lineageos/setupwizard/FinishActivity.java

@@ -111,7 +111,6 @@ public class FinishActivity extends BaseSetupWizardActivity {
         sendBroadcastAsUser(i, getCallingUserHandle(), FINISH_SETUP);
 
         setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_LOCKED);
-        hideBackButton();
         hideNextButton();
         finishSetup();
     }