DecorView: Fix status guard

Fixes several issues with the status guard:
- poor background color for light status bar background
- guard & action mode need insets for the navigation bar in landscape
- status guard was not properly dismissed after completion of the action mode
- uses the proper non-zero root inset when LAYOUT_HIDE_NAVIGATION is unset & in landscape

Bug: 132924796
Test: make ApiDemos, use SystemUiModes activity to trigger action mode
Change-Id: Ie9ba2b6fb2c87a1825df85f756ffb062a45d4f55