Add ime back button logic to the init block of NavbarButtonsViewController

Fixes: 338320015
Test: Run adb shell am start -a android.intent.action.MAIN -n com.google.android.setupwizard/.SetupWizardTestActivity. Make sure the back key shows by default but hides when IME renders back arrow.
Change-Id: I8baeb10d3f5b13c3bac084872d77d881c1513733
(cherry picked from commit 4b43cfbc)