Don't send the opaque appearance if the window hides the bar

This CL considers that the window behind a system bar draws behind the
bar if the window requested to hide the bar. And DisplayPolicy doesn't
need to send the opaque appearance flag for the type to System UI.
Otherwise, the user will see a redundant black background of the bar
while it is hiding.

Fix: 395510336
Flag: EXEMPT bugfix
Test: Launch a floating tranlucent activity which hides system bars, and
      see if the system bar background stays transparent.
Change-Id: Ibc759bb8d9b5f2ad4ee0967a174269e39484f6bf