Fix 3 button mode issues in AllSetActivity

- Set system bars appearance to light/dark depending on device
  theme to ensure visibility
- Also fix issue where taskbar icons would appear partially
  offscreen and prevent pressing nav buttons if you rotated

Fixes: 293438087
Test: adb shell am start -a android.intent.action.MAIN -n com.google.android.setupwizard/.SetupWizardTestActivity
  - Skip to the end (AllSetActivity), and ensure 3 buttons are visible
  - Rotate device and ensure 3 buttons are still visible and functional
  - Will also flash / wipe to the build once it's created
Flag: None
Change-Id: Ie2d9bf1e1d95a6ccbd1f9c592adb51390b21ad85