Get correct insets of the phone screen at boot

Before places such as
DockedStackDividerController.initSnapAlgorithmForRotations() tries to
run getStableInsetsLw() will get no insets on boot (via
onConfigurationChanged) because the navigationbar and statusbar have not
been added yet. This would leads to some miscalculations until
configuration changes such as rotating the screen.

Test: manual - logged the inset values on bootup with adb logcat
Change-Id: If07c8c007cb355dc418ff987fbb36ce104192379