Update hasNavigationBar when shouldShowSystemDecors changes

This CL updates DisplayPolicy.mHasNavigationBar and
NavigationBarControllerImpl.mHasNavBar when shouldShowSystemDecors
changes.

Bug: 352461502
Bug: 374240163
Flag: com.android.server.display.feature.flags.enable_display_content_mode_management
Test: manually (adb shell settings put secure mirror_built_in_display {1|0})
Change-Id: I8700a500661aa4b5f3cb836ac0c8c9c8694bfd93