Fix issue #5371530: SYSTEMUI_FLAG_HIDE_NAVIGATION reasserts itself immediately

This cleans up how ui flags are managed between the client and window manager.
It still reports the global UI mode state to the callback, but we now only clear
certain flags when the system goes out of a state (currently this just means the
hide nav bar mode), and don't corrupt other flags in the application when the
global state changes.

Also introduces a sequence number between the app and window manager, to avoid
using bad old data coming from the app during these transitions.

Change-Id: I40bbd12d9b7b69fc0ff1c7dc0cb58a933d4dfb23