Add margin to SUW back button

* navButtonsLayoutParams marginEnd was removed, and
SUW was relying on that value to add margins to
SUW back button, but was returning 0.

Fixes: 242999939
Test: Visually observed in SUW test activity
adb shell am start -n com.google.android.setupwizard/.SetupWizardTestActivity

Change-Id: I5ffc421e9b731b0d9c7b7c48eab807d4419721b1