Fix how we hide and show the nav bar.

The PhoneWindowManager is now responsible for hiding and showing
the nav bar.

For hiding, it just moves it off the screen (easy way to get a
nice slide animation on and off).  At the same time, we use a
new WM facility to put up a fake input window to capture all
touch events.

When a touch event is received, we force the system UI to clear
the navigation hiding bit so it will be shown again.

This removes a bunch of code from the system UI for hiding and
showing the nav bar.  Also removes the code calling from userActivity()
to the system UI, which was bad.  (Also no longer using userActivity()
fixes bugs around re-showing the nav bar due to key presses and
other wrong things.)

Change-Id: I8c3174873b5bcaa36a92322a51e8f7993e88e551