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