Don't flash system bars while showing an immersive app on lockscreen

Before applying this patch, when a show-when-locked immersive app is
showing, the system bars would quickly show and hide, which are
redundant to the user.

The root cause is that, for nav bar, we have a policy to show nav bar
if the width and height of status bar are MATCH_PARENT and status bar
has no PRIVATE_FLAG_KEYGUARD. When keyguard is becoming status bar,
its keyguard flag would be removed first, and then the height would
be changed to the bar height. So the nav bar would be shown between
these events. For status bar, we force showing it when it is expanded
by checking its width and height are MATCH_PARENT or not.

To fix the issue, this change introduces a new private flag which
indicates that the status bar window is now in an explicit expanded
state. We check this flag instead of checking the width and height of
status bar.

This change also fix a bug that: when AOD is enabled, if the
foreground app has FLAG_SHOW_WHEN_LOCKED, FLAG_TURN_SCREEN_ON, and
FLAG_FULLSCREEN, clicking on the power key would make it show the app
again instead of AOD. (not 100%, but chances)

Bug: 80147982
Test: 1. go/wm-smoke
      2. Launch a show-when-locked turn-screen-on immersive app on
         AOD, and see if any system bar flashes.
      3. Launch a show-when-locked turn-screen-on immersive app on
         lockscreen, and see if any system bar flashes.
      4. a. Enable AOD in Settings.
         b. Open a show-when-locked turn-screen-on immersive app.
         c. Click on power key, and see if AOD shows.
      5. Launch an immersive app and drag down the status bar, see
         if nav bar keeps there as long as status bar is expanded.

Change-Id: Ie885d504eb73ae8a86736b2c3ed4fb03eb9f739e