Fix nav bar visibility on lock screen

StatusBarKeyguardViewManager was assuming that the navigation bar
was the root view in its window, which it isn't anymore. Add a
getRootView() to ensure it is always the root.

Change-Id: Ifeeecc1ae8d7b6c4235a1ce4767dad3919bf90e6
Fixes: 34452399
Test: lock device, tap on nav bar area