Fix back button not touchable in SUW

Since NavbarButtonsViewController already checks visibility of buttons before adding them to the touchableRegion, we can remove teh isThreeButton() check.

Test: run adb shell am start -a android.intent.action.MAIN -n com.google.android.setupwizard/.SetupWizardTestActivity, then press back button and ensure it gives visual feedback (even though there's no page to go back to in this test activity).
Bug: 191846922
Change-Id: I10d3d046cd2d14017638a2c71ea6b9fe228fadfc