Only enable up button if device is provisioned

Test: Carry out QR provisioning, observe that no more back button
      in the nav bar of wifi picker page
Test: After SuW flow,
      Run adb shell am start -a android.net.wifi.PICK_WIFI_NETWORK
      Observe there is a back button in the nav bar.

Change-Id: I50ec5b2c465eee87f5a7906e3353a0070c65e2ee
FIX: 74059636

src/com/android/settings/SettingsActivity.java

@@ -308,8 +308,9 @@ public class SettingsActivity extends SettingsDrawerActivity
 
         ActionBar actionBar = getActionBar();
         if (actionBar != null) {
-            actionBar.setDisplayHomeAsUpEnabled(true);
-            actionBar.setHomeButtonEnabled(true);
+            boolean deviceProvisioned = Utils.isDeviceProvisioned(this);
+            actionBar.setDisplayHomeAsUpEnabled(deviceProvisioned);
+            actionBar.setHomeButtonEnabled(deviceProvisioned);
             actionBar.setDisplayShowTitleEnabled(!mIsShowingDashboard);
         }
         mSwitchBar = findViewById(R.id.switch_bar);