Show back button in SUW

- Listen for USER_SETUP_COMPLETE
- Recreate taskbar, forcing nav buttons to be visible

Test: Run setup wizard and ensure back button is present, then is replaced with taskbar when setup is complete
adb shell am start -a android.intent.action.MAIN -n com.google.android.setupwizard/.SetupWizardTestActivity

Fixes: 191846922
Change-Id: I789186dcc23a2db7cc4637b1cb0486f0d2add784