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
  open emergency dialer and check the key pad reacts
Change-Id: I7876c92331125b00931218d9e5aca164a1ea50d9