Keep FLAG_SHOW_WALLPAPER flag on NotificationShade.

Before this change, we controlled wallpaper visibility on the Lockscreen
by combination of visibility of NotificationShade window and
setting/resetting FLAG_SHOW_WALLPAPER flag on its window. When the flag
was removed while NotificationShade was still visible (e.g. OCCLUDE
transition starts on the Lockscreen), the wallpaper became immediately
invisible and caused a flicker.

With this change, we keep the FLAG_SHOW_WALLPAPER on NotificationShade,
and only rely on its visibility weather the wallpaper should be visible
or not on the Lockscreen.

Bug: 191438572
Test: existing tests + manual (open GPay app from the lockscreen)
Change-Id: Id13ffbbd2c965e0dcc97effd3faf46fd7a8bb1a5