Only force drawing status bar background if target >= N

Since forcing it all the time has the potential of breaking
compatibility with apps, we don't want to do this.

Instead, we only force it if the app targets >= N.

We communicate this to window manager with
PRIVATE_FLAG_FORCE_DRAW_STATUS_BAR_BACKGROUND.

We introduced this for 2-up split-screen. If we have an app
that doesn't draw the status bar background by itself, we
just force the whole bar to be black.

The same applies for windows that used translucent status
bar - we also force the whole bar to be black

Bug: 27285627
Change-Id: I7f1ceaa364f8a4e851935f77aa5e8d913bf11791