Fix taskbar layout issues in setup wizard

- Align nav buttons (only back is enabled) to start instead of end
- Don't animate from init()
- Provide 0 contentInsets.bottom
  - Auto-stash the taskbar during setup
  - Hide the stashed handle by adding an alpha channel for home disabled
  - Report 0 contentInsets when stashed if the handle isn't visible
- Tint nav buttons according to theme

Test: adb shell am start -a android.intent.action.MAIN -n com.google.android.setupwizard/.SetupWizardTestActivity
Bug: 194786060
Change-Id: I4a40501e8aad2a38ec00398efe9ea3dbfa7428cd